328 PALEONTOLOGY OF ISEW YORK. 



mental, the latter is wholly secondary ; a replacement of, but never a derivative 

 from the former. In the foregoing discussions of the genera these parts have 

 been distinguished simply by the designations generally current ; the term 

 deltidium referring exclusively to the convex external portion of the pedicle- 

 sheath, such as occurs in Clitambonites, Strophomena, Rafinesquina, and their 

 allies, and which, under no condition, shows evidence of composition or con- 

 solidation of separate parts. The term deltidial plates has been applied to that 

 condition of the external sheath in which a division into component parts is 

 evident, as in Athyris, Atrypa, Merista, the terebratuloids, etc. ; or inferen- 

 tial, as in CvRTiA and Cyrtina. The terminology is here so imperfect as easily 

 to cause confusion, and though it had not seemed needful heretofore to suggest 

 an improvement, it has become necessary, for the proper consideration of the 

 subject, to employ a more distinctive expression for these fundamentally differ- 

 ent structures. The secondary structures known as the deltidial plates, whether 

 already discrete as in the terebratuloids, rhynchonelloids and meristoids, or 

 solidly coalesced, as in Nucleospira, Parazyga, Cyrtia and Ctrtina, will hence- 

 forward be termed the ddtarium, m application to the parts as a wliolo, or the 

 deltaria in referring to the component plates. It may also prove convenient to 

 adopt the terra introduced by Broxn, pseudodeltidium, for the coalesced condi- 

 tion of the deltaria in Spirifer, Cyrtia, etc., as this is its original meaning ; 

 but the significance of the term will be subordinate to that of deltarium. 



The researches by Kowalevski,* upon the development and detailed anatomy 

 of Thecidea (Lacazella) and Cistella ( = Argiope, Kow.), have recently been 

 interpreted in the bearing upon these structures by Beecher, who has also added 

 new data derived from the study o? M igellania flavescens and Terebratulina septen- 

 trionalis. Thecidea, or Lacazella Muditerranea, is the latest and only existing 

 brachiopod which retains a true deltidium at maturity. Daring the cephalula- 

 stage of the embryo, before the inversion of the mantle lobes to enclose the 

 head, two shell-plates begin to form, one on the inner side of the dorsal mantle 

 lobe, the other directly opposite to it on the outer surface of that portion of the 



* Obse7-vations on the Development of the Bi-achiopoda; Proceeding-s of the Session of the Imperial 

 Society of Amateur Naturalists, etc., held at the University of Moscow, Eleventh year, vol. xiv. 1874. 



