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879 



TERTIARY FAUNA OF FLORIDA 



rad is based on V. lintea, a small species of the same general type, though 

 perhaps more strictly referable to the subgenus Cymbophora referred to below. 



Section Oxyperas Morch, 1853. 

 Type M. i7-iangula7'is Lam. 



This section is characterized by its more crudely triangular shape and 

 rather strong concentric sulcation of the surface of the valves. It is chiefly 

 Indo-Pacific in its distribution. 



Subgenus LEPTOSPISULA Dall, 1895. 



Type Mactra striatella Lam. 



Shell thin, inflated, with undulated beaks, the dorsal areas smooth ; 

 pallial sinus large, deep ; gape well marked ; valves convex ; ligament sagit- 

 tate, dental armature concentrated ; opposed surface of the laterals smooth ; 

 anterior arm of left cardinal coalescent with the lateral ; anterior arm of right 

 cardinal coalescent with the dorsal lamina ; hinge-plate thin and excavated ; 

 spur prominent, but the chondrophore not roofed at the ape.x. 



This group in the Spisuloid division represents the MactrelLa type in the 

 Mactroid section. The type of the subgenus is said to come from India. 



Subgenus CYMBOPHORA Gabb, 1869. 

 Type Mactra Ashburiieri Gabb. 



A careful study of the typical species of this group shows that it differs 

 from Spisula only in the following features. The attached ends of the re- 

 silium were convex instead of flat (as is sometimes seen in recent species), and 

 the margins of the pit are therefore elevated ; while the posterior sinus, instead 

 of being (as usually in the later types of Spisidd) roofed over or filled up with 

 a solid mass of callus at the apex, upon which the ligamejit is attached, is 

 vacant, so that the ligament was fixed on the convex margin of the pit, or on 

 the side of the ventral lamina, or partly on both, all being very close together. 

 This character would seem to be trifling until it is observed that all the Meso- 

 zoic species are characterized by this feature, though, as in recent Spisula, the 

 external form may vary, the dorsal areas be smooth or grooved, the teeth 

 sulcate or smooth. As it is common to all the Cretaceous Mactridie of which 

 I have been able to examine a hinge, I have thought it best to retain the 

 name in a subgeneric sense for that stage of development of the group. 

 Gabb's figures are too formalized and do not bring out the features clearly. 



