TROPIDOLEPTUS OF THE HAMILTON GROUP. 
407 
The exterior extremity of the cardinal process presents considerable variety of 
aspect, when a large number of individuals are examined. In some of them this 
part, if stripped of the external callosity of pseudo-deltidium, would have the 
main process bilobed, with a sinus a little below the apex, and an accessory lobe 
on each side similar to some of the species cf Productus. 
Tropidoleptus carinatus. 
PLATE LXII. 
Strophomma car inala : Conrad, Ann. Geol. Rep. p. 64. 1839. 
Leptccna laticosta ■: Hall, 1843. . 
“ «< of Owen and others. 
Tropidoleptus carinatus : Hall in Tenth Report on the State Cabinet, p. 151. 1857. 
Tropidoleptus : Genus described and illustrated in Twelfth Report on the State Cabinet, p 31. 1859. 
Shell concavo-convex, semielliptical, the length sometimes nearly 
equalling the width : hinge-line equalling, greater, or less than the 
width of the shell, and the cardinal extremities sometimes rounded 
so as to give the shell a broadly oval form ; the sides are sometimes 
nearly straight, and the front broadly rounded. 
Ventral valve convex, broadly subcarinate along the middle, and sloping 
in a flattened curve to the lateral. margins and front, which is some¬ 
times slightly truncate or emarginate : cardinal extremities deflected, 
abruptly incurved at the umbo, and the apex often imperfect from the 
encroachments of the foramen. Area from half a line to about one line 
in width; its margins parallel to near the extremities, where it slopes 
suddenly down from the outer margin. The area is longitudinally 
striate, and indented by a very wide foramen. 
Dorsal valve moderately concave, sometimes nearly flat, often with a 
median depression or sinus which becomes conspicuous below the 
middle of the valve : apex small, projecting a little beyond the hinge- 
line. There is a narrow area, interrupted in the middle by a wide 
pseudo-deltidium which covers the extremity of the cardinal process. 
Surface marked by* about eighteen to twenty broad, simple, rounded 
plications which are wider than the spaces between them; the cen¬ 
tral one on the ventral valve is broader and more elevated than the 
^others, while there is a corresponding wider and deeper depression in 
