16 Prof. W. King on Spirifer cuspidatus. 



Beginning with Spirifer cuspidatus^ I am enabled to give, 

 in PL II. fig. 11, a representation of a specimen (the one 

 already noticed as belonging to Mr. Morton, F.G.S. &c.) which 

 shows the inside of the arch, with the dental plates, free from 

 the infilling of foreign matter which generally conceals it. 

 The arch extends for about two-thirds of the length of the 

 deltoid fissure, gradually rising from its origin at the apex of 

 the umbone to its free extremity, where it is nearly a quarter 

 of an inch in height : in the centre of the margin of the arch 

 there is a slight projection, which corresponds to the termina- 

 tion of the canal, as shown in Winchell's figure of his so-called 

 Syringothyris typa^. Owing to the arch and the median fold 

 or sinus of the pertaining valve passing so far into the cavity 

 of the shell, a comparatively small space, necessarily, inter- 

 venes for the animal. 



By grinding down the umbone of the large valve of a 

 number of specimens, I have succeeded in finding the apo- 

 physis in every one of them. The canal and septum vary in 

 different specimens, even in the same individual. Occasionally 

 the canal is enclosed in the septum, the middle of which is 

 enlarged by it (fig, 12, PI. III. : this belongs to No. 3 spe- 

 cimen). It also occurs attached to the outer or upper side of 

 the septum (fig. 13, No. 4 sp.) ; or it projects from the inner 

 or under side (fig. 15, No. 5 sp.). Generally the septum is 

 concave to the plane of the area (fig. 13) ; occasionally it is 

 parallel with (fig. 12), or convex to it (fig. 15). 



The fullest information respecting the canaliferous septum 

 has been revealed to me by operating on Professor Harkness's 

 imperforate specimen. The diagram under fig. 16 represents 

 a lateral view of one of the dental plates (a), also a vertical 

 section of the transverse septum {h) and its canal (c), drawn to 

 a scale. The valve to which these structures belong is 2\ 

 inches in height. The dental plates stretch right across the 

 umbonal cavity, from its vertex to a level about midway be- 

 tween the apex and the hinge ; at this level the plates are 

 about an inch in width. Adjoining the area their length is 

 the same as the height of the valve ; but, owing to their free 

 margin having a deep concave curve, they suddenly decrease 

 -to an inch and one-eighth at a point about two-thirds across 

 the umbonal cavity f ; they gradually lengthen again in ap- 

 proaching the inner surface of tlie valve, getting about a 

 quarter of an inch longer. 



* See Davidson's paper in 'Geological Magazine,' July 1807, pi. 14. 

 fig. 4^. 



t I find tte dental plates to yavy somewhat in length at the vertex of 

 the curve in different specimens. 



