﻿COSTATiE. 



155 



The typical form ovately trigonal, short, very convex at the position of the marginal 

 carina ; umbones elevated, pointed, much arched inwards, and somewhat recurved ; 

 anterior side short, its border truncated, lengthened, depressed at the junction of the 

 valves, its lower portion curved elliptically with the lower border, which is short and 

 nearly straight ; hinge-border very convex and short, forming a considerable angle with 

 the siphonal border, which is equal to it in length and is excavated at its upper or anal 

 portion. Escutcheon raised, convex, cordate; its breadth in the united valves is equal to 

 three fourths of its length ; it is well circumscribed by a prominent indented inner carina, 

 and has a series of large, closely arranged, obliquely diverging, dentated, but depressed 

 costellae. Area very large ; together with the escutcheon it is equal in size to the other 

 portion of the valve, with the surface of which it forms nearly a right angle ; its greatest 

 breadth measured upon the siphonal border exceeds half the height of the entire valve ; it 

 is bounded outwardly by a large, deeply indented, marginal carina ; a small but well-defined 

 median carina divides it into two nearly equal portions ; the superior portion is depressed 

 and concave ; it has a numerous series of minute, delicate, oblique, reticulated costellae ; 

 the other or outer portion of the area has in the right valve only one or two large indented 

 costellae ; the median carina in its lower portion usually divides into two similar costellae ; 

 the lower portion of the other valve has four or five costellae. The marginal carina of 

 the left valve somewhat overwraps the ante-carinal groove ; in the right valve the post- 

 carinal groove is conspicuous, and the marginal carina is much larger than that of the 

 other valve. The transverse striations upon the costellae of the area are minute and 

 delicate — a feature which affords a contrast to the more deeply sculptured indentations 

 upon some other species of the Costatce. The other portion of the surface has the costae 

 large, elevated, short, and only slightly oblique in their general direction ; anteally they 

 have a small, sudden undulation at the curvature of the valve, and become attenuated 

 near the border; their number in adult forms varies from eighteen to twenty-seven. 

 In the left valve their posteal extremities end suddenly at the border of the ante-carinal 

 groove, where each forms a slight enlargement ; in the right valve they pass onwards 

 and are united to the marginal carina. 



Dimensions of an adult specimen. — Height 30 lines ; diameter of a valve at right 

 angles to the marginal carina 21 lines ; across the area of the united valves 21 lines ; 

 length of the escutcheon 15 lines; its breadth 9 lines. 



There is great uniformity in the surface-ornaments in specimens of different states of 

 development, belonging to the typical form ; the area and escutcheon more especially are 

 almost without variation, and differ only in the convexity of the escutcheon, thus 

 rendering the hinge-border either horizontal or oblique. Specimens with the valves 

 united often have them perfectly closed by the oblique opposition of the extremities of 

 the marginal carina, thus indicating the exertion of muscular power when they were 

 overwhelmed by a muddy current unfitted to be introduced into the gills by the incurrent 

 orifice. 



