INTRODUCTION. 9 



from all others of the genus. Agassiz placed T. duplicata, Sow., with the Scabra ; it is here 

 placed with the Scaphoidea for reasons which are given with the description of that species. 

 The Scabra, including the two sub-groups above indicated, constitute, perhaps, the most 

 prominent and characteristic fossils of the Cretaceous formations ; from whatever part of the 

 world they are obtained both their natural history and geological position admit of no 

 dispute, so nearly do the American and Asiatic species approach to the more well known 

 of the European forms. 



7th. — Costa™. 



In the preceding sections the opposite valves present no permanent or systematic 

 differences either in their figure or ornamentation ; in the present section, on the contrary, 

 the difference of the valves in both particulars is universal and nearly uniform in their kind, 

 varying chiefly in their degree of prominence or otherwise ; separate descriptions of the 

 valves, therefore, becomes necessary. 



The Costatse constitute a numerous and almost entirely Jurassic section recognized by 

 longitudinal elevated plain costae upon the sides of the valves ; it also possesses other not 

 less persistent and distinctive sectional features. The area is well separated from the 

 costae ; it is bounded in each valve bv two well marked dentated carinas, and is divided 

 into two nearly equal portions longitudinally. The superior portion is more depressed than 

 the other ; it is divided longitudinally by a small furrow, bordering upon which and placed 

 upon the lower portion is usually a small indented median carina ; these two inter-carinal 

 spaces have small longitudinal indented costellae, which take the same oblique direction 

 as the carina. In the condition of advanced growth the entire ornamentation of the area 

 is usually replaced by transverse irregular plications of growth. The escutcheon varies 

 greatly in its relative size and figure in the outline of its upper border, and not less so in 

 its ornamentation, so that it constitutes an important feature in the Costata, without 

 which the definition of any of its species is incomplete and insufficient. The area in the two 

 valves presents some well marked differences ; the lower or carinal half of the rigid valve 

 has the inter-carinal costellae fewer and larger than those upon the other valve ; they are 

 also irregular and unequal, so that commonly the median carina only exists as one of these 

 larger costellae, of which there are usually from two to four. There is also a deeply ex- 

 cavated groove upon the area immediately posterior to the marginal carina and partly over- 

 lapped and concealed by it; this may be termed the post-carinal sulcus ; in the other 

 valve a similar sulcus exists immediately anterior to the marginal carina, which it separates 

 from the extremities of the costae. It is more conspicuous than the sulcus in the other valve, 

 and is the ante-carinal sulcus ; its use corresponds to that of the post-carinal sulcus in the 

 other valve ; the open extremities of the marginal carinas when the valves were in opposi- 

 tion formed an aperture or incurrent orifice of the gills which admitted of being perfectly 



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