16 Prof. W. King on Spirifer cuspidatus. 
Beginning with Spirifer euspidatus, 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, Pl. 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 (4) and its canal (c), drawn to 
a scale. The valve to which these structures belong is 24 
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{; they gradually lengthen again in ap- 
proaching the inner surface of the valve, getting about a 
quarter of an inch longer. 
* See Davidson’s paper in ‘Geological Magazine,’ July 1867, pl. 14. 
g. 4é. 
+ I find the dental plates to vary somewhat in length at the vertex of 
the curve in different specimens, 
