328 
PALEONTOLOGY OF NEW 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, Stropiiomena, 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 Cyrtlna, will hence¬ 
forward be termed the deltarium, in application to the parts as a whole, or the 
deltaria in referring to the component plates. It may also prove convenient to 
adopt the term 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 of Migellania Jiavescens and Terebratulina septen- 
trionalis. Thecidea, or Lacazella Mediterranea, is the latest and only existing 
brachiopod which retains a true deltidium at maturity. During 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 
* Observations on the Development of the Bi-achiopoda; Proceedings of the Session of the Imperial 
Society of Amateur Naturalists, etc,, held at the University of Moscow, Eleventh year, vol. xiv. 1874. 
