SILURIAN BRACHIOPODA 57 



while the form (r) is more rarely met with. The form (^) is also of remark- 

 able persistence, and starts so early in the life of the individual as to sug- 

 gest a distinct genetic impulse. 



V^ariations 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 development of the sinal plications. A 

 marked illustration of this is afforded by an individual which, in repairing 

 an injury to its shell, has abruptly developed six plications on one side of 

 the sinus, in continuation of three and to correspond with three on the 

 opposite side. 



The absence of plications in the sinus. This is a feature of 

 rare occurrence, and is undoubtedly an infantile character retained 

 in later stages of growth. A single individual of immature growth 

 affords an illustration of a peculiar abnormality, indicating a reversal 

 in the growth to an embryonic condition. This shell (plate 5, figs. 

 3, 3«) has grown to a certain size and normally developed its plications, 

 but an abrupt period has been placed to their development, and over the 

 entire anterior portion of the individual, in front of a stOut varix, the sur- 

 face of the shell is almost smooth. This is the exact counterpart of that 

 mode of growth, to which attention is called under other species, where 

 the smooth embryonic condition of the shell seems to be prolonged for 

 more than the usual period of immaturity, and the mature features are 

 thereupon abruptly developed after the formation of a sharp growth-line. 



DEVELOPMENTAL VARIATIONS 



Our series of individuals illustrating the embryological changes in this 

 species is so complete as to show by almost imperceptible gradations the 

 entire chain of development from very near the starting point up to matur- 

 ity. This series begins with an individual measuring i mm. in length and 

 8 



