SILURIAN BRACHIOPODA 95 



usually are found to unite with the line of the normal form ((5), several 

 removes from the initial member of the series. Sometimes the varieties 

 do not reach the adult dimensions of the normal shell and may be con- 

 sidered as varietal dwarfs. 



The representation of varietal and of certain abnormal conditions by 

 complete series of fossil specimens shows, that in these directions, there 

 was a distinct developmental tendency, or genetic impulse, independent of 

 normal growth. 



Senility i^lways expressed by the thickening of the shell which takes 

 place after the individual reaches adult size. The thickening may involve 

 the whole interior of the valves, producing a truncate appearance at the 

 margins, or it may take place by frequent interrupted growth along the 

 margins, giving to this portion a varicose aspect. As a result of this senile 

 growth, the vertical diameter of the shell is increased, and the beaks 

 involuted, so that they are often so closely appressed as to conceal the 

 cardinal area and truncate the ventral beak, and in addition, the margins 

 of the valves lose the characteristic ornamentation of the species and cor- 

 respond to the clinologic and nostologic stages as defined by Mr. Hyatt.* 



Abnormalities frequently find an explanation in some pathological or 

 accidental conditions which become instituted at any period in the life of 

 the animal, and leave their impress on the shell. The functional failure 

 of a developing organ may cause the parts to revert to an embryonal type, 

 and although it is difficult to apply this statement to the shelly covering 

 of the animal, yet this condition is sometimes found. The specimen of 

 Rkynchonella neglecta described on page 37 is an instance of this 

 kind. Another abnormal variation is noticed in certain individuals which 

 preserve the larval features of the shell long after it has passed the early 

 stages, and when, in many cases, it has reached the full adult dimensions. 



* Proceedings of the Boston Society of Natural History, Vol. xxiii, 1888 ; Values in Classi- 

 fication of the Stages of Growth and Decline, with propositions for a New Nomenclature. 



