82 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



whether any of the species seen by him have a projecting plate connected 

 with the posterior muscular scar; but perfectly preserved internal casts of 

 the following-described species, believed to belong to this group, show no 

 indications of such a process, there being on them only a faintly-impressed 

 hair-line, margining both the anterior and posterior scars, and scarcely visible 

 without the aid of a magnifier. 



This genus is also nearly related to Cucullaria of Deshayes; but differs 

 from that group as typified by Area hctcrodonta, Desh., in having a longer, 

 straighter hinge-margin, and straighter and more regular anterior and posterior 

 teeth, with less-developed denticles under the beaks. Its general form is also 

 more like that of Area, while that of Cucullaria has the outline of Barbuda, 

 Gray. 



Mr. Conrad thinks this genus confined to the Cretaceous; but there are 

 also some Jurassic, and perhaps older species, that approach very nearly to 

 its characters — such, for instance, as Cucullcm Parkinsoni, as figured by 

 Quenstedt (Der Jura, II, pi. 67, fig. 14), from the Jura. 



It is quite probable that many persons will think it wholly unnecessary 

 to notice such slight differences as distinguish this type from Macrodon and 

 Cucullaria; but if we attempt to divide the whole of the Arcida of all ages 

 into genera and subgenera, it must be evident to any one who will attentively 

 study the numerous extinct types, that very slight differences must be taken 

 into account, or else we must admit that the whole vast group shades into a 

 single genus. 



\< motion SMlcatillUS, Evans & Slium. isp.i 



Plate 15, figs. 6, a, 1>. 

 Area sulcatum, Evans and Sbumard (18- r >7), Trans. Acad. Sci. St. Louis, 1, 39. 



Shell small, transversely rhombic-trapezoidal, about half as high as long; 

 beaks rather depressed, and placed a little in advance of the middle, incurved 

 and somewhat distant; posterior umbonal slopes oblkjue, prominent or sub- 

 angular; cardinal margin straight, ecfualing about five-sixths the length of the 

 valves; basal margin parallel to the dorsal, nearly straight, or more or less 

 sinuous near the middle; anterior margin rounding up a little obliquely, so as 

 to intersect the hinge above at slightly less than a right angle; posterior 

 margin truncated from the abruptly-rounded or subangular posterior basal ex- 

 tremity, a little obliquely forward and upward, with a slightly sinuous outline, 

 so as to connect with the end of the hinge at rather more than a right angle; 

 cardinal area unknown: Fren margins finely crenate within; internal casts 



