92 MEMOIRS OF THE STATE MUSEUM 



late species, but agreeing closely with some of the inarticulate genera, c, g. 

 Lingula, Lcptobolus, Obolus. The specialization which accompanies subse- 

 quent growth confines the pedicle more closely to the ventral aperture, and, 

 as a result, the dorsal aperture is gradually filled by a callosity. Thus, 

 also, the Strophomenidee ; but Streptorhynchus subplanum shows at maturity 

 what has not yet appeared in Or this, viz., 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 Strophomenidae here discussed for facts confirmatory 

 of our 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 Strophodonta, 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 subapical sheath. In both Strophodonta and Streptor- 

 hynchus (especially of the later paleozoic faunas), the cavity of the pedicle- 

 sheath, if it be retained in any form, at maturity, 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 development of the deltidial 

 plates at maturity. Such is Rhynchonella to a large degree ; also Cistclla, 

 Argiope, Kraussina, Terebratclla, and Magasella ; and we may assume the 

 structural degeneracy which is thus indicated, to be 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 



