CTETOCEEATID^. 
275 
Sp. Char. Shell strongly curved. Upon a chord of 70 millim., 
subtending the concave side, the highest perpendicular is 15 millim. 
But in several examples the shell shows a tendency to become 
straighter in the upper part of the more mature individuals. The 
cross section is an ellipse, of which the ventro-dorsal is to the 
transverse diameter as 5 : 6. The rate of increase is in the ratio of 
about 2 to 5 in a length of 60 millim. of the septate part of the 
shell, hut it varies a little in different individuals. The body- 
chamber is verj’ nearly equal in length to the ventro-dorsal diameter 
of the shell at its aperture, or about J the total length of the shell. 
There is a series of very regularly spaced crenulations at the base of 
the body-chamber. A slight sinus presents itself on the convex 
border of the aperture. The septa are distant from each other in 
adult individuals about ^ the shell-diameter ; the number of septal 
chambers may be estimated at about 60 in an adult shell. The 
siphuncle is situated close to the margin of the convex border of the 
shell, its elements are inflated between the septa. The ornaments of 
the test consist of longitudinal ridges, so strong as to appear upon 
the cast; they extend throughout the whole length of the shell, 
without any intermediate lesser ridges (except in very young shells) ; 
they are from 16 to 22 in number ; the spaces between them are a 
little concave. The ridges are crossed at pretty regular intervals by 
transverse imbricating lamellm, which form very prominent frills 
standing out from the surface of the test. In the intervals between 
the ridges the lamellae form little festoons whose convexity is 
directed towards the aperture. The lamellae are rarely preserved in 
a calcareous matrix, but in the slaty rocks of Dworetz they are found 
intact. The spacing of the lamellae varies considerably in different 
individuals. 
Remarlcs. Closely allied to this species is the Cyrtoceras tredecimaley 
of Phillips (Pal. Boss, of Cornwall &c. p. 114, pi. xliv. ff. 215, a-d ) ; 
but in the latter the ornamentation, though of a similar character, is 
coarser, there being only 13 longitudinal ridges in the circumference 
of the shell, and these are elevated at distant intervals, so that they 
form a series of encircling ridges, giving a boldly angulated appear- 
ance to the outline of the shell. The Cyrtoceras acutocostatum of 
G. and P. Sandberger (Die Yerstein. J^assau, p. 144, Taf. xiii, ff. 5, 5a) 
seems to he identical with C. tredecimale. Phillips indeed mentions 
that his species is found in the Eifel. 
Horizon. Etage E (= Salopian). 
Localities. Prague, Dworetz, Bohemia. 
Well represented in the Collection. 
