SILURIAN BRACHIOPODA 93 



leading through Strophomcna, Scenidium, Orthisina, Lcptcena, C/ionetes, 

 Prodiictus, and Stropkalosia, and the other in the direction of Rkynchonella, 

 Spirifer, A try pa, Retzia, and Terebratnla. 



INTERNAL APPARATUS. Our observations upon the development of 

 the brachial supports are limited to the species Retzia evax. Here it is 

 found that the number of revolutions of the spiral ribbon increases with 

 age, but we are not prepared to say what the inceptive condition of this 

 apparatus may have been. In the early stage represented on page 60, 

 where the ribbon has completed two revolutions, the supports must have 

 been exceedingly tenuous and delicate, for they can be traced in the crystal- 

 line or muddy filling of the shell, only by extremely faint lines, composed of 

 minute dots of pyrite. As observed under the discussion of these features, 

 the character or actual existence of the loop connecting the spirals was not 

 established, but it is developed, with all normal characters, in a shell 4 mm. 

 in length, where the ribbon makes four revolutions. 



It has been shown by MORSE,* that in Tercbratulina septentrionalis, the 

 loop (i. e. the entire brachial support) begins by the development of two 

 acute processes from the lower moiety of the dental plate, which assume 

 the character of crura, eventually meeting and coalescing on the dorsal side, 

 forming the completed loop at an early stage, the ventral horns of the loop 

 never uniting. The simple nature of the support in these shells precludes 

 the possibility of the continued growth which obtains in the more compli- 

 cated apparatus of the spiriferous species. The inception of the brachial 

 support was observed by MORSE in an individual i mm. in length, but the 

 lateral processes are not conspicuously developed until a length of 3 mm. is 

 attained, and they have not united at a length of 4 mm. It is therefore 

 possible, from these data, that Retzia cvax does not have the loop com- 

 pleted at so early an age as that indicated by a length of 2.5 mm. 



* Memoirs of the Boston Society of Natural History, Vol. ii ; On the Early Stages of Tere- 

 bratulina septentrionalis, pi. 2, figs 48-55. 1869. 



