TROPIDOLEPTUS OP 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 bilobcd, witli a sinus a little below the apex, and an accessory lobe 

 ou each side similar to some of the species of Pboductus. 



Tropuloleptas carinatus. 



PLATE LXIL 



Strophomena carinata : Conrad, Ann. Geol. Hop. p. 64. 1839. 

 Leptana lalicatta : Hall, 1843. 



•< << of Owen and others. 



Tropidoleptui carinatut : Hall in Tenth Report on the State Cabinet, p. 151. 1857. 

 Teopidileptcs : 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. 



DoRS.\L 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 



