SIPHONIDA. 



497 



362) has the posterior slope of the shell radiately ribbed, 

 while the rest of the shell is concentrically striated. Hemi- 

 cardium has keeled valves, the shell appearing cordate as 

 viewed from behind or in front. Lunulicardium comprises 

 a number of Devonian Cockles, in which there is a deep 

 excavated "lunule" beneath the beaks. Lymnocardium 

 and Adacna include brackish-water and fresh-water Cockles, 

 in which the cardinal teeth are small or obsolete ; and the 

 species of the former are common in some of the fluviatile 

 and estuarine deposits of the Upper Tertiary period. 



Lastly, we must place here the extraordinary genus Cono- 

 cardium ( = Pleurorhynchus), which ranges from the Silurian 

 to the Carboniferous. The shell in this type (fig. 363) is 



Fig. 3t53. A, Conocardium, giganteum, showing the truncate anterior and produced posterior 

 end of the shell, with the tubular prolongation of the former ; B. Conocardium inflatum, 

 viewed from above ; c, The same viewed laterally. Carboniferous. (After M'Coy.) 



keeled, and very oblique, the anterior end of the shell being 

 short and abruptly truncated, so as to appear as a cordate 

 flattened area when the shell is viewed from the front. 

 Just below the beaks the shell is produced anteriorly into a 

 VOL. I. 2 I 



