XI] 



THE SHELLS OF PTEROPODS 



837 



of growth, instead of a maximum on the one side and a minimum 

 on the other side of our tubular shell. As a matter of fact, the 

 dorsal and the ventral plate tend to curve in opposite directions, 

 towards the middle line, the dorsal curving ventrally and the ventral 

 curving towards the dorsal side. 



In the case of the Lamelhbranch or the Brachiopod, it is quite 

 possible for both valves to grow into more or less pronounced spirals, 

 for the simple reason that they are hinged upon one another; and 

 each growing, edge, instead of being brought to a standstill by the 

 growth of its opposite neighbour, is free to move out of the way, 

 by the rotation about the hinge of the plane in which it Hes. 



But where there is no such hinge, as in the Pteropod, the dorsal 

 and ventral halves of the shell (or dorsal and ventral valves, if we 



Fig. 415. Pteropod shells, from the side: (1) Cleodora cnspidafa; (2) Hyalaea 

 longirostris; (3) H. trispinosa. After Boas. 



may call them so) would soon interfere with one another's progress 

 if they curved towards one another (as they do in a cockle), 

 and the development of a pair of conjugate spirals would become 

 impossible. Nevertheless, there is obviously, in both dorsal and 

 ventral valve, a tendency to the development of a spiral curve, that 

 of the ventral valve being paore marked than that of the larger and 

 overlapping dorsal one, exactly as in the two unequal valves of 

 Terebratula, In many cases (e.g. Cleodora cuspidata), the dorsal 

 valve or plate, strengthened and stiffened by its midrib, is nearly 

 straight, while the curvature of the other is well displayed. But 

 the case will be materially altered and simplified if growth be arrested 

 or retarded in either half of the shell. Suppose for instance that 

 the dorsal valve grew so slowly that after a while, in comparison 

 with the other, we might speak of it as being absent altogether: 



