XI] 



OF BIVALVE SHELLS 



567 



Among the bivalves the spiral angle (a) is very small in the 

 flattened shells, such as Orthis, Lingula or Anomia. It is larger, 

 as a rule, in the Lamellibranchs than in the Brachiopods, but in 

 the latter it is of considerable magnitude among the Pentameri. 

 Among the Lamelhbranchs it is largest in such forms as Isocardia 

 and Diceras, and in the very curious genus Caprinella ; in all of 

 these last-named genera its magnitude leads to the production of 

 a spiral shell of several whorls, precisely as in the univalves. The 

 angle is usually equal, but of opposite sign, in the two valves of 

 the Lamellibranch, and usually of opposite sign but unequal in 



Fig. 290. Caprinella adversa. 

 (After Woodward.) 



Fig. 291. Section of Productus 

 (Stroj}home)ia) sp. (From 

 Woods.) 



the two valves of the Brachiopod. It is very unequal in many 

 Ostreidae, and especially in such forms as Gryphaea, or in Capri- 

 nella, which is a kind of exaggerated Gryphaea. Occasionally it 

 is of the same sign in both valves (that is to say, both valves curve 

 the same way) as we see sometimes in Anomia, and much better 

 in Productus or Strophomena. 



Owing to the large growth-factor of the generating curve, and 

 the comparatively small angle of the spiral, the whole shell seldom 

 assumes a spiral form so conspicuous as to manifest in a typical 

 way the helical twist or shear which is so conspicuous in the 



