DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 361 



Ventral valve with the beak much elevated above the 

 dorsal, and incurved, so that the plane of the foramen is 

 parallel to the axial plane of the shell. Foramen circular, or 

 slightly sub-triangular ; deltidial plates generally obscure on 

 account of tlie infolding of the beak. 



Dorsal valve regularly arcuate, except at the posterior 

 extremity, where the beak is closely incurved beneath the 

 ventral umbo. The median sinus usually carries from three to 

 five plications, but in advanced growth sometimes becomes 

 filled up by the crowding of these plications. Ventral valve 

 with a well-marked sinus, generally bearing three plications. 

 The sinal plications on both valves take their origin in front 

 of the beak, and are of interstitial growth, a fact which does 

 not hold true for any of the other plications. The surface is 

 marked by from eight to twelve rounded, continuous plica- 

 tions on each side the sinus of either valve, all of these 

 extending to the beak, with the possible exception of the 

 more obscure ones on the cardinal slopes. Only in rare 

 instances and abnormally do these plications increase by 

 interstitial addition. Imbricating lines of growth are often 

 present, and fine concentric striee are sometimes discernible. 



The mature individuals of Homoeospira evax divide them- 

 selves into three groups, based on their relative proportions : 



(a) Normal form, in which the length and width are equal. 



{h) Long form, in which the length is greater than the 

 width. 



(c) Broad form, in which the length is less than the width. 



In frequency of occurrence the form (J) almost equals the 

 normal, while the form (c) is more rarely met with. The 

 form (5) is also of remarkable persistence, and starts so early 

 in the life of the individual as to suggest a distinct genetic 

 impulse. 



Variations from the Normal Development. — These are to an 

 unusual degree very slight, and may be classed as follows : 



Obesity, which apparently occurs only when normal full 

 growth has been attained. 



A tendency to asymmetry in the development of the sinal 



