) ^ \ 



64 G. C. Cnclc — On Cyrtocerasfrom Derbyshire. 



and based upon examples from St. Doulagb's, co. Dublin, Ireland, 

 was cbaracterized as follows : — 



" Sbell of moderate size, rather sbarpl}' curved in the lower third 

 of the septate portion, but becoming almost straight above this, so- 

 that a fragment consisting only of the upper two-thirds of the shell 

 would scarcely exhibit any curvature. Upon a chord of 38 mm. 

 subtending the concave side of the apical region of tbe shell the 

 greatest curvatui^e is 5 mm. The rate of tapering above this curved 

 part is about 1 : 4, which is a rapid increase in diameter. The 

 section is very nearly circular, the siphuncle close to the margin of 

 the outer curvature of the shell. The body-chamber considerably 

 exceeds one-third of the length of the entire shell ; its basal line is 

 indicated by the letters a, h, in fig. 2 of pi. xii, in which its obliquity 

 is very mai'ked, making an angle with the horizontal axis of the 

 shell of about 18°. The septa are numerous, and being tilted up in 

 a ventro-dorsal direction the sutures have a strong obliquity on the 

 sides of the shell, while they are nearly horizontal on the ventral 

 and dorsal aspects, perhaps with a slight arching upwards on the 

 dorsal aspect (pi. xii, figs, la, 16). The distance between the 

 sutures in an adult shell (pi. xii, fig. \h), where the diameter is 

 about 50 mm., is from 5 to 6 mm. ; in a somewhat smaller example 

 (pi. xii, fig. 3) the sutures are 4 mm. apart where the diameter is 

 15 mm., 6 mm. apart where it is 37 mm. In a smaller specimen 

 (pi. xi, figs. 2a, 25), which is entirely septate, there are twenty-two 

 septa within a distance of 106 mm. The chambers must thus have 

 been very shallow. Exactly in the median line of the ventral 

 aspect, or outer curvature of the shell, there is a straight thread-like 

 line or keel, feebly developed, but clearly perceptible when the 

 surface of the cast has not been abraded ; it is represented rather too 

 broad in the figure (pi. xii, fig. 16). The siphuncle is exogastric — 

 that is, it is situated close to the convex or ventral border of the 

 shell ; it is strongly inflated in passing through the chambers 

 (pi. xii, figs. 3, 4a, 46), casts of it presenting the characteristic 

 bead-like appearance as in the last figures referred to. 



" The surface of the shell is perfectly smooth. 



''The approximate measurements of the most complete specimen 

 give length 190 mm., greatest diameter of body-chamber 53 mm., 

 diminishing to 48 mm. at or close to the aperture, and 6 mm. near 

 the apical extremity." 



The specimen in Dr. Wheelton Hind's collection from Kniveton 

 consists of the internal cast of the greater part of the body-chamber, 

 and of the ventral portion of the four preceding chambers (see 

 Plate III, Figs. 1-4, p. 63). The body-chamber has a mean length 

 of 73 mm., and is slightly curved ; it attains its greatest diameter at 

 about one-third of its length from the aperture, the ventro-dorsal 

 and transverse diameters here being 43 and 44 mm. respectively ; 

 it gradually tapers both towards the aperture, where its ventro- 

 dorsal and transverse diameters are 40 and 42 mm. respectively, and 

 towards the base of the body-chamber, which is imperfect dorsally 

 but has a transverse diameter of 34 mm. Part of the aperture is 



