50 CEETACEOUS PELECYPODA 



be distinctly indicated. The lutrauiinm are, like the myin^, only known from 

 cainozoic deposits, a great number of the miocene forms being very similar to, or 

 identical with, recent species. Already in the lower tertiary, or eocene, beds the 

 presence of their species is doubtful, with the exception of the forms designated 

 by Conrad Fteropsis, 



There have been species described under the name of Lutraria from cretaceous, 

 Jurassic, and by McCoy even from carboniferous strata; but the generic deter- 

 mination is by no means certain; it was based merely upon external resem- 

 blance; and the Jurassic species, so called, have already been sufficiently proved 

 not to belong to the lutrariinm at all, being mostly species of various genera of 

 the Anatinibje, 



Of the Mactrinje there are about 60 tertiary species well known, and typical 

 species, though very few, of this sub-family certainly occur in cretaceous beds, but 

 those from older formations are as yet doubtful ; at least the hinges of none of those 

 described from Jurassic and older deposits are known, and the external form is, as 

 in the previous case, not to be depended upon as a point of reliable generic 

 determination. 



On account of this scarcity of species in a fossil state I shall have but few 

 genera to record in addition to those which are treated of in most of the larger 

 conchological works. The generic divisions, mostly introduced by Dr. Gray, are, 

 however, in several instances based upon too minute distinctions, and require some 

 alterations. I am by no means certain whether it is at all necessary to distinguish 

 some of them even by a separate sub-generic name, as I shall state subsequently. 



a. 8ul-famihj—L UTBABIINJi]. 



The arrangement of the ged^ra of this sub-family will be 1st, according to the 

 prevalent development of the cardinal teeth next to the cartilage-pit, 2nd, to that of 

 the laterals, and so on. 



1. Anatinella, Sow., 1829. Shell oval, posteriorly somewhat produced and 

 gaping; hinge with a long, oblique, posteriorly directed spoon-shaped process in 

 each valve for the reception of the cartilage; next to the cardinal process there are 

 two teeth on each side in the right valve, in the left valve there is a bifid tooth in 

 front next to the margin of the process, and a very indistinct posterior one near the 

 upper margin; lateral teeth obsolete, a very short external ligament just behind 

 the beaks is also present, pallial impression entire, anterior muscular iiSpression 

 elongated, marginal, posterior roundish. ^ 



Anatinella is in many respects allied to Tugonia; it has a similar form, a cartil- 

 age process in each valve, a simple pallial line, etc., but it has unquestionably the 

 distmctive character of the lutrahiinje, possessing similar small teeth next to the 

 cartilage process as seen in Lutraria. It is, so to say, equally related to Tresus as is 

 Tugonia to Flatyodon or Mya, but until the animals are known the systematic 

 position of both the genera must remain somewhat unsettled. 



