34 CARBONIFEROUS CEPHALOPODA OF IRELAND. 



ment cousisting ouly of the upper two-thirds of the shell would scarcely exhibit 

 any curvature. Upon a chord of 08 uim. subtending the concave side of the 

 apical region of the shell the greatest curvature 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, b, in fig. 2 

 of PI. XTI, in which its obliquity is very marked, 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 veuti-al and dorsal 

 aspects, perhaps with a slight arching upwards on the dorsal aspect (PI. XII, 

 figs. 1 a, 1 h). The distance between the sutures in an adult shell (PL XII, 

 fig. 1 b), where the diameter is about 50 mm., is from 5 to G mm. ; iu a somewhat 

 smaller example (PL 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 (PL XI, figs. 

 2 a, 2 b), which is entirely septate, there are twenty-two septa within a distance 

 of 106 mm. The chambers must thus have been very shallow. Exactly iu 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 (PL XII, fig. 1 b). 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 (PL XII, figs. 3, 4 a, 4 b), casts of it presenting the 

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

 The surface of the shell is perfectly smooth. 



St2e. — 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. 



Affinities. — There can be no doubt that Cijrtoccras rostratum, de Koninck,^ is 

 closely related to the presont species. Both forms have quickly tapeiing, thick 

 shells, with the curvature most marked in the apical region. Tlie distinguishing 

 characters are found chiefly in the septa, which ai'e strongly oblique in C. apicale, 

 while they are ouly very slightly so in C. rostratum. The section also in the latter 

 species is distinctly oval (cf. de Koninck, PL xxxv, fig. 1 b), while it has been 

 shown to be almost circular in the former. The oval form of the section iu de 

 Koninck's species is brought out in his figures, in which the narrow ventral aspect 

 is in strong contrast with the much broader lateral one. The section gives a 



' ' Fauue Calc. Carb. Belg.,' IhSO, toiii. v, p. 26, \<\. xxxv, tigs. 1, 2. 



