SPIRIFER. 23 



represented in fig. 10, but not generally so much indented, in most cases they are scarcely 

 visible. 



If we now separate completely the two valves, the interior will be seen as illustrated 

 by figs. 13 and 15. In the larger valve, fig. 15, we perceive, on each side of the deltideal 

 fissure, two teeth, which fit into corresponding sockets in the smaller valve, forming a strong 

 hinge, so that the valves cannot be separated except by breaking one of them ; they are 

 placed at the extremity of two dental plates, projecting into the shell, forming the sides or 

 walls of the deltideal fissure, and extending to the beak, which they strengthen (figs. 15 and 

 16). Between the two dental plates, a central system is interposed, variable in thickness 

 and development, as may be seen in figs. 15, 16; beginning by a thick basis, which gradually 

 decreases till it becomes as sharp as the edge of a knife, and projecting far beyond the 

 lateral or dental plates (figs. 16, 21), two muscular impressions are visible between the 

 dental and central plate. The interior of both valves is closely punctuated. 



From the discovery by Mr. Moore, of specimens completely freed from all matrix, in 

 which the most minute delicate impressions and details are preserved as intact as if the 

 animal had just left its shell, I am able to offer a much more complete description of the 

 small valve than that given in my paper published in the 'London Geological Journal,' (1845). 

 This valve, separated completely from the larger one, would appear as in fig. 1 3 ; we first notice 

 the dental sockets and position of the calcareous supports ; when both valves are united they 

 fill the greater portion of the larger valve, except where it covers the teeth and hinge, it will 

 be seen that two lamella issue from a strong basis under the sockets which extend, dimin- 

 ishing in width and thickness, till they reach about the middle of the shell, where a curved 

 lamellar process unites the two spires, but which process is rarely perceptible except when 

 the specimens are in a perfect state of preservation (figs. 16, 21); the two lamella again 

 continue to be directed toward the front of the shell, diverging from one another as they 

 advance, and finally turning towards the bottom of the large valve, forming the first and 

 successive coils, known under the name of spirals, each circle diminishing in circumference 

 and size as it approaches the sides of the shell. The spire has been, for Mr. Moore and 

 myself, the subject of active researches, and having found some specimens full of a very 

 fine sand, it was preserved in great perfection, and, as may be remembered, in 1847 

 I mentioned the presence of spines on the spire, but at that epoch we could not offer 

 observations as complete as at present. The lamella which forms the spire is neither smooth 

 nor of equal thickness on all its width, differing on each side and variable, but always thicker 

 on the inner side of the circumference than on the other which tapers out into an acute 

 edge, and as will be seen in figs. 17, 18, the thickest part of the spire is towards its middle, 

 where it forms a circular elevation diminishing again towards the outer edge. 



It will be observed that no spines ever appear on the face of the lamella fronting the 

 sides of the shell, or on the internal edge of the spire, as is observed in figs. 17, 18 ; the 

 spines only occur in that part of the spire facing the front of the shell where it opens, 

 covering thus only about a quarter of the circumference of each coil. These spines 



