Cuming s — Morphogenesis of Platystrophia. 131 



separated ; in the latter they are closely approximated. In the 

 same way we may compare Pentagonia and Meristella. 



The extent of the brachia may also bear some definite rela- 

 tion to the width of the shell.* Thus it might be reasonably 

 expected that in such a form as P. acutilirata the spires would 

 have a greater number of volutions than in such a form as P. 

 costata. The analogy of the Spirifers holds here also ; for in 

 S. mucronatus there are often just as many turns to the spirals 

 as plications on the shell. The Spiriferacea indeed suggest 

 that there may be a more or less definite correlation between 

 the presence and number of plications and the extent and 

 position of the spires of the brachia. 



The subject of the cause of plications is an exceedingly 

 tempting one, but leads too far afield into the realm of theory 

 to warrant extended discussion here. A few suggestions may 

 nevertheless be hazarded. 



The relation of the brachia to the mantle in structure and 

 function is intimate, since the former are derived from the lat- 

 ter, and both cooperate in the function of respiration. f Some 

 definite relation between the volumes of the two is, therefore, 

 to be expected. Increase in the extent of the brachia is 

 accomplished by folding and coiling of various sorts, while 

 increase in mantle area is accomplished by radial growth (in- 

 crease in the absolute size of the shell) and much more effect- 

 ively by plication. The cooperative efficiency of the brachia 

 and mantle can, therefore, for a shell of given size, be greatly 

 increased by coiling or folding of the brachia and plication of 

 the mantle. The correlation may be even more precise : for 

 where the axes of the spires lie in the transverse diameter of 

 the shell the position and direction of the successive volutions 

 is such that the mantle may conform to them in shape, fold- 

 ing up and down as it crosses each turn. 



There are other factors that may have been concerned in 

 causing or accentuating shell plication. The greatly increased 

 length of the mantle border of a plicated shell as compared 

 with a smooth shell of the same size, is obvious. Such a pli- 

 cated shell could admit the respiratory and food-bearing cur- 

 rents of water, and at the same time keep out foreign particles 

 and disagreeable visitors, because the greatly lengthened slit 

 between the mantle margins would not necessitate so wide a 

 gaping of the valves of the shell. That there actually is a 

 correlation between the gape of the valves and the presence of 



* Beecher has pointed out the effect of mechanical restraint in the produc- 

 tion of transverse shells (this Journal, vi, 1898, p. 337). Doubtless this 

 factor was concerned in the great extension of the cardinal angles of P. 

 acutilirata. 



f The brachia are not directly respiratory organs, but indirectly, by caus- 

 ing the flow of currents of water over the mantle. 



