MESOCHASMOCERAS LATIDORSATUM. 73 



emend. Hyatt; Strohoceras, Hyatt; Apheleceras, Hyatt; Mesocliasmoceras,VooYdi ; 

 Suhclymenia, d'Orbigny ; Diorugoceras, Hyatt (of doubtful validity, the single 

 species [infra, p. 74] upon which it is founded being very imperfectly known). 



Mesoohasmocebas LATIDORSATUM, F. M'Go>/, sp. Plate XX, figs. 14 a, b, 15 a — d. 



1844. Nautilus latidobsatus, F. M'Coi/. Synop. Carb. Foss. Ireland, p. 18 



pi. iv, fig. 16. 



Description. — Shell discoid, much compressed, with about two and a half 

 volutions, the sides of which are entirely exposed in a large and very shallow 

 umbilicus which has a remarkably large central vacuity. The section of the 

 whorl, scutiforin in the young, becomes in the adolescent stage of growth sub- 

 quadrate and tetragonal, the adult becoming more compressed from side to side 

 and hence more elongate dorso-ventrally. The zone of impression is slight and 

 its boundaries faintly defined. The initial point of the apex is not seen, but the 

 shell is present in one individual examined up to a very short distance from it, 

 and measures where broken off 1*5 mm. in its dorso- ventral diameter, and 1 mm. 

 in the transverse direction. The section of the young shell is nearly ovate, the 

 angularities afterwards developed not being present at this stage. Before the 

 first whorl has been attained the periphery becomes flattened, the dorsal or anti- 

 peripheral area, however, still remaining rounded, and it is not until the second 

 wliorl has begun that a faintly defined lateral angle or shoulder begins to be 

 formed ; this gradually becomes more distinct, and constitutes a definite, though 

 never sharply defined angle to the umbilicus, whence the whorl slopes down to 

 the suture formed by contact with the preceding whorl. The periphery which is 

 narrow now becomes concave, but not deeply so, and it is bordered on each side 

 by an inconspicuous keel. The sides, after the young stage of the shell is past, 

 are flattened, or slightly convex, a faint depression being formed in the adolescent 

 (neanic) stage near the keel, giving the latter more prominence at this part of the 

 shell. The overlapping of the whorls does not extend beyond the peripheral margin. 



The body-chamber occupies about half of the last volution. 



The septa are moderately distant, and the curvature of their sutures indicates 

 that the chambers were rather deep (PI. XX, fig. 14 b). The sutures are 4*5 mm. 

 apart where the transverse diameter of the whorl from the edge of the periphery 

 to the suture of the shell measures 12 mm., this distance being reduced to 3 mm. 

 where the diameter, similarly measured, is 11 mm. 



The siphuncle is situated slightly above the centre of the septum. 



The surface of the test is smooth, only some obscure folds being present here 



12 



