TRANSACTIONS OF WAGNER 

 886 



TERTIARY FAUNA OF FLORIDA 



In order to compare Trcsus with another group to which it has been 

 affiliated, a specimen of Mya truncata from Bering Sea was examined. The 

 difference here was more decided ; so far as general appearances go the two 

 animals looked very different, but an analysis of the differences does not 

 reveal anything very striking. The gills in Mya were smoother than in either 

 of the Mactroids, and their anchorage to the siphonal septum was more like 

 that of Spisnla than Trcsus. The foot in Mya is much reduced. The palpi 

 are smaller and much less adherent to the mantle; the mantle is much more 

 closed ventrally and the gills are more posterior as a whole than in cither of 

 the Mactroids. There is no elevated osphradial raphe apparent in the Mya. 

 In other features all three genera seemed pretty much alike. 



Through the kindness of Dr. Nolan, secretary of the Philadelphia 

 Academy, I have received information showing that the signature of the 

 Academy's Proceedings which contained the description of Schhotluents by 

 Conrad was published about the end of, January, 1853, while Gray's Trcsns 

 appeared in the January number of the Annals of Natural History, which 

 was doubtless issued in the first days of the month. Tresus will therefore 

 take precedence. The distinction attempted to be drawn between Trcsus and 

 Schizothcerus by Conrad in his Catalogue of Mactridcs is without sufficient 

 basis in fact. There is but one species, which varies (like Mya) considerably 

 in form and proportions. It is found on both sides of the North Pacific and 

 fossil in California. 



Genus STANDELLA Gray, 1853. 



Type Mactra fragilis Gray non Chemnitz = M. pellucida (Ch.) Gmelin, 1788; not 

 Standella H. and A. Adams, 1856. 



Shell short, subequilateral, thin, compressed, dorsal areas obscure; sur- 

 face striated or vermiculate; beaks low, adjacent; pallial sinus deep; siphonal 

 gape moderate ; ligament not set off from the chondrophore by a shelly 

 septum; dental armature concentrated; chondrophore moderate, oblique; 

 resilium homogeneously continuous between the valves ; left cardinal wide, 

 prominent, with a very small posterior accessory lamella; anterior lateral 

 short, high, adjacent to the cardinal ; posterior lateral longer, both well de- 

 veloped and partly confluent with the ventral edge of the hinge-plate; right 

 cardinal wide, the posterior arm walling the pit, the anterior arm superposed 

 on the ventral lamina; both lamina; present before and behind the pit; the 

 posterior sinus in the right valve distinctly roofed. 



Owing to a confusion between two of Chemnitz's figures, by which his 



