XI] 



OF BIVALVE SHELLS 



569 



direction of each of the two branches, parallel to the valve (or 

 nearly so) and outwards from the middle line, will tend to con- 

 stitute a curve of double curvature, and so, on further growth, 

 to develop into a helicoid. This is what actually occurs, in the 

 great majority of cases. But the curvature may be such that 

 the heUcoid grows outwards from the middle line, or inwards 

 towards the middle line, a very slight difference in the initial 

 curvature being sufficient to direct the spire the one way or the 

 other; the middle course of an undeviating discoid spire will be 

 rare, from the usual lack of any obvious controlling force to prevent 

 its deviation. The cases in which the helicoid spires point towards, 

 or point away from, the middle line are ascribed, in zoological 

 classification, to particular "famihes" of Brachiopods, the former 



Fig. 293. Spiral arms of 

 Spirifer. (From Woods.) 



Fig. 294. Inwardly dii-ected 

 spiral arms of Atrypa. 



condition defining (or helping to define) the Atrypidae and the 

 latter the Spiriferidae and Athyridae. It is obvious that the 

 incipient curvature of the arms, and consequently the form and 

 direction of the spirals, will be influenced by the surrounding 

 pressures, and these in turn by the general shape of the shell. 

 We shall expect, accordingly, to find the long outwardly directed 

 spirals associated with shells which are transversely elongated, as 

 Spirifer is ; while the more rounded Atrypas will tend to the 

 opposite condition. In a few cases, as in Cyrtina or Reticularia, 

 where the shell is comparatively narrow but long, and where the 

 uncoiled basal support of the arms is long also, the spiral coils 

 into w^hich the latter grow are turned backwards, in the direction 

 where there is room for them. And in the few cases where the 

 shell is very considerably flattened, the spirals (if they find room 



