DEVELOPMENT OF SOME SILURIAN BRACHIOPODA 395 



namely, the initiation of deltidial plates, in conformity with 

 the general course of development of the cardinal features 

 observed in other families. 



It is not well in this place to go beyond the scope of this 

 work, and the species of Strophomenidse here discussed for 

 facts confirmatory of these observations. It may be remarked 

 that the stage at which the development of the deltidial 

 features has been arrested at maturity in this family varies 

 with the species, not with the genus. When every trace of 

 these features is obliterated, as is usual in Stropheodonta^ a 

 slight abrasion of the apical substance of the shell will often 

 show a trace of the obsolete pedicle-tube. At times, in the 

 same genus, this is retained at maturity as an external feature, 

 and in such a case is usually accompanied by some indication 

 of the sub-apical sheath. In both Stropheodonta and Ortho- 

 thetes (especially of the later Paleozoic faunas), the cavity 

 of the pedicle-sheath, if it is letained in any form, at matu- 

 rity, has been filled by the deposition of calcareous matter 

 about the compound cardinal processes of the opposite valve, 

 and thus wholly diverted from its original function. 



In conclusion, it is to be observed that of recent species of 

 brachiopods a very great number show an incomplete devel- 

 opment of the deltidial plates at maturity. Such is Rhynclio- 

 nella to a large degree ; also Qistella, Kraussina, Terehratella^ 

 and Magasella ; and it may be assumed that the structural 

 degeneracy which is thus indicated is the natural concomitant 

 of the secular decline of the entire class. 



It is not improbable that from an early form related to the 

 genus Orthis, phylogenetic development tended in two main 

 channels, — one leading through Strophomena, Scenidium, 

 OrtJiisina, Leptcena, Chonetes, Productus, and Strophalosia, 

 and the other in the direction of Rliynchonella, Spirifer, 

 Atri/pa, Retzia, and Terehratula. 



Internal Apparatus. — The present observations upon the de- 

 velopment of the brachial supports are limited to the species 

 Homceospira evax. Here it is found that the number of revo- 

 lutions of the spiral ribbon increases with age, but it is not 



