FAMILIES OF LOOP-BEARING BRACHIOPODA 305 



of reproduction, by the influence of environment, threw off 

 genera which did not go through the complete series of meta- 

 morphoses, but are otherwise fully adult, and even may show 

 reversional tendencies due to old age ; so that nearly every 

 stage passed through by the higher genera has a fixed repre- 

 sentative in a lower genus. Moreover, the lower genera are 

 not merely equivalent to or in exact parallelism with the 

 early stages of the higher, but they express a permanent type 

 of structure, as far as these genera are concerned, and after 

 reaching maturity do not show a tendency to attain higher 

 phases of development, but thicken the shell and cardinal 

 process, absorb the deltidial plates, and exhibit all the evi- 

 dences of senility and reversion presented during the old age 

 of the higher genera. Kraussina shows a partial loss of del- 

 tidial plates and a thickened septum, with two strong prongs 

 representing the ascending branches, which in other genera 

 are very delicate. Bouchardia, Magasella (M. Cumingi), and 

 Agulhasia have excessively thickened hinge-plates and mus- 

 cular fulcra ; the deltidial plates are obsolete, but the pedicle 

 is enclosed by the growth and overlapping of the edges of 

 the delthyrium. Megathyris and Cistella deposit lime on the 

 interior of the valves, often in the form of nodes and ridges, 

 while the pedicle in its growth encroaches upon the beaks of 

 both valves to the final elimination of the deltidial plates. 

 Platidia represents the extreme of this encroachment. Thus 

 there are incomplete types of brachial development in these 

 genera, accompanied by positive evidences of senility and 

 retrogression in other shell characters. 



Classification. 



In the light of the previous discussion, the classification of 

 the families and genera of Ancylobrachia, with the exception 

 of the fossil forms, is comparatively simple. Among the 

 extinct genera of the TerebratulidsB, or the family in which 

 the loop is a development of the descending branches, Cen~ 

 tronella and Eensselceria belong to one sub-family; Stringo- 

 cephalus to another; and Megalanteris, Cryptonella, Dielasma^ 



DictyothyriS) and others, will come in the Terebratulinse. 



20 



