[62] On the structure of shells the reader should consult the admirable papers of Dr. Carpenter, in the British Association Reports.
Fig. 5. Terebratula coarctata, of Parkinson. Bradford clay, Wilts.
Figs. 6, & 7, show the internal structure of recent Terebratulæ from New Holland. The complicated shelly apophyses which supported the arms are quite perfect.
Fig. 8. Terebratula triquetra, of Parkinson (T. diphya, of Lamarck); another example of the species, fig. 4.
Figs. 9, & 10. Different parts of the same specimen of a brachiopodous bivalve belonging to the genus Productus, so named from the lengthened or produced form of the convex valve. "This is generally filled with limestone, which conceals the internal structure; but, with a slight blow, the shell divides, when the edge of the small valve rests against the inside of the produced cylindrical part of the larger one; generally about half an inch from the top of the shell: one side of the valve, before hidden, fig. 9 a, is then exposed, as shown in fig. 10."—Mr. Parkinson.
Fig. 9. a, the beak of the upper valve; c, a cavity in the superior part of the shell.
Fig. 10. The under part of the shell; b, a depression receiving the beak of the upper valve, a.
Fig. 10*. The inner surface of another upper valve, having a longitudinal fissure. The species figured is the Productus Martini of Mr. Sowerby. From the mountain limestone of Derbyshire; in which deposit numerous examples occur.
Fig. 11. a large species of Spirifer (Spirifer striatus, of Sowerby), from the mountain limestone of Derbyshire. In this species the upper valve is broken away, and one of the large spiral apophyses is seen lying imbedded in the limestone with which the cavity of the shell is filled.
Fig. 13, is a beautiful example of part of one of the spiral appendages of the same species.