Lister, pl. 433. f. 276., and App. pl. 19. f. 1, 2. Pennant, pl. 42. f. 1. Da Costa, pl. 16. f. 2. Donovan, pl. 118. Wood, pl. 13. f. 1, 2, 3. Dorset Cat. pl. 3. f. 2.

These are references to the works of British authors in which this shell is represented by a figure. The first reference is to the great work of Dr. Martin Lister, entitled "Historia, sive Synopsis Methodica, Conchyliorum," in the 433d plate of which the figure marked 276. is the present shell, and also figures 1. and 2. of the 19th plate of another work of the same author, entitled "Appendix Historia Animalium Anglie." In each of the books we have selected, you will find lists of authors who have written on Conchology; and therefore, when you meet with a reference which you do not clearly understand, turn to these lists, and you will there ascertain the name of the author, the title of his works, the place and year when published, &c. In Turton's Dictionary the account is limited to British authors; but in Burrow's Elements and Dillwyn's Catalogue you will find an alphabetical arrangement of the writers on conchological science, whether British or foreign. Dr. Turton gives some account also of the works mentioned by him, and some occasional observations respecting their authors.

For instance, you wish to have some idea of the work of Lister, first referred to above, you therefore look for the word "Lister” in Burrow's List of Writers on Conchology, and you find the following:

"LISTER, MARTIN. Historia, sive Synopsis Methodica, Conchyliorum. Lond. 1685—1692. Oxon. 1770. à GULIELMO HUDDESHORD, with copper-plates."

But in Dr. Turton's work you find the following interesting remarks on this same work:

"Lister. Huddesford's edition, Lister's Historia Conchyliorum. Folio, 1770. The basis and ancient foundation of all good Conchology. This admirable volume contains one thousand and fifty-five plates, besides twenty-one of anatomical figures; all drawn from original specimens by his two daughters, Susanna and Anna. Considering the state of natural science at the time this work was first issued, one hundred and thirty-three years since, it is impossible to contemplate this stupendous effort of genius and industry, without admiration at the grandeur of the design, and the correctness of its execution. Some of the plates, especially the anatomical ones, are of matchless excellence. And it is gratifying to recollect, that the original drawings are preserved among the archives of

the University of Oxford. His Historia Animalium Angliæ, and its Appendix, are now of rare occurrence."

If you examine the account of this species in Dillwyn, you will find a much longer list of references, these not being there limited to British writers. And though at first you may have some little difficulty in understanding them, yet by a little practice in examining the lists of authors I have alluded to, you will soon get acquainted with these abbreviations, and know to what books they relate.

The description of the Piddock given by Turton is too long for my present purpose, and therefore I will adopt Dillwyn's. In page 35. of his work, you will find this specific character: "Shell oblong, with reticulated striæ ; and the anterior end strongly muricated and beaked."

And, as is usual in systematic arrangements, there is, after the references and synonyms, a farther description, thus: "Shell about an inch and a quarter long, and four or five inches broad, thin, brittle, and white; the exterior surface is covered with reticulated striæ, which gradually become stronger and more prickly towards the anterior end." I have marked with italics the terms which I think you will here require to be explained. What is the anterior end of a

bivalve shell (for the Pholas, with the exception of the accessory valves, is in all other respects a bivalve)? Fig. 26. will explain. The part marked (a) is a tough elastic horny substance,

which connects the two valves on one side of the summits or bosses (umbones) of the shell (b), and is named the Ligament or Cartilage. This is so constituted, that, by its elasticity, the valves have a constant tendency to open. They are kept shut by the contraction of a muscle inside; and hence, when, in opening an oyster or a scallop, we cut through that muscle the shells

separate to a considerable distance. When the animal also is so long out of its native element, that it is thereby greatly weakened, and the contractile power of the muscle diminished, we observe that the animal lies with the shells open, the elasticity of the ligament being too powerful for the debilitated contractility of the muscle. Now the part where the cartilage or ligament is placed is the anterior; while, of course, the other side is the posterior, end.

You may next, perhaps, think it rather singular that the shell should be described as so much broader than it is long; but you must learn, that the length of a shell means the distance from the hinge to the base, and, consequently, the length may either be the longer or the shorter diameter, according to the species. The muscle, for example, (fig. 26. c.) is as long again as it is broad; while in the pod razor shell (Solen Legumen) (fig. 26. d.) the breadth is nearly four inches, and the length scarcely three quarters of an inch. The length of a shell, then, does not mean its longest diameter, but the space from the hinge to the base; and, consequently, it varies exceedingly; in some shells being longer, in some much shorter, and