204 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



the hinge-plate, or a spoon-shaped cavity raised above the same ; pallial sinus 

 rounded or angular. 



The above diagnosis is drawn up so as to include three groups that have 

 been regarded by some as constituting distinct genera. This may be so; but 

 I have preferred to unite them here as subgroups of the same genus, not only 

 because the distinctions do not seem to be very important, or always strongly 

 marked, but because it is practically impossible to distinguish between them, 

 in most cases, among fossil species ; while there may be some doubts which 

 should retain the name Mactra, in case they should be separated as distinct 

 genera. They may be separately defined as follows : 



1. mactra, Linn, (typical rz Trigonella, da Costa"). 



Shell trigonal, or subtrigonal ; ligament separated from the car- 

 tilage-cavity by a shelly ridge ; pallial sinus rounded. — (Example as 

 already stated.) 



2. cymbophora, Gabb. 



Differs from the last chiefly in having its cartilage-cavity spoon- 

 shaped, with its prominent margins raised above the hinge-plate, and 

 the A-shaped cardinal tooth of the left valve more distinct from the 

 margin of the cartilage-cavity. — Mactra Ashburnerii, Gabb. — (Appar- 

 ently an entirely Cretaceous group.) 



3. schizodesma, Gray. 



Shell trigonal ; ligament connecting with the upper part of the 

 carlilage-cavity ; pallial sinus angular. — Mactra stultorum, Linn. 

 The question in regard to which of the species originally included by 

 Linnjeus in his genus Mactra, should be viewed as the typical form, has 

 given origin to some confusion in nomenclature. If all of the species included 

 by Linnaeus belonged to one genus, it would, of course, be a matter of very 

 little importance which one should be taken as the type. Unfortunately, 

 however, this is not the case, and his diagnosis (as usual) is so short and 

 indefinite as to give no aid in deciding this point; while he did not mention, 

 or otherwise designate, which species he viewed as the typical form. Some 

 high authorities in natural history, in such cases, always take the first species 

 mentioned by an author as the type of a genus. If we were to follow this 

 rule rigidly, we would have to regard Mactra Spengleri, Linn., as the type of 

 the genus, which would make it exactly equivalent to Gray's more recently 

 proposed group Scliizodcsma, which I have retained above as a subgenus. 



