256 STUDIES IN EVOLUTION 



writer are correct, it is derived from an ancestry which had a 

 similar condition of fixation as early as the Upper Silurian. 

 Thecidium is apparently not a terebratuloid genus. Its struc- 

 tural affinities are evidently with the strophomenoids, espe- 

 cially such forms as Plectambonites, Leptoenisca, etc. Briefly 

 the reasons for this statement are (a) the presence of a del- 

 tidium of one plate ; (6) the absence of a true loop supporting 

 the arms (the internal calcification or spiculization is confined 

 wholly to the mantle, and does not extend to the arms 16 ) ; 

 (c) a concave plate in the cavity of the ventral beak, bearing 

 the divaricator muscles ; (d) the attached ventral valve, and 

 (e) the cardinal processes in the dorsal valve.* The first 

 character is of prime importance, because all the strophom- 

 enoids and none of the terebratuloids have a deltidium of 

 one plate. 



It would appear, therefore, that the early, free-swimming, 

 larval state, and the later pediculate stage have become lost 

 by acceleration, thus accounting for the very unequal develop- 

 ment of the mantle lobes in the cephalula stage, and the non- 

 active and early sedentary larvae as described by Kovalevski 

 and Lacaze-Duthiers. 



The young Lingula (G-lottidia) described by Brooks, and 

 the Discinisca by M tiller, 20 both representing the phylem- 

 bryonic stage, were active and free-swimming animals, with 

 rudimentary pedicles. Terebratulina becomes attached or 

 rests on the caudal segment during the cephalula stage 

 (Morse), while at the end of this period in Cistella (Kova- 

 levski and Shipley) there is an active, swimming, ciliated 

 organism, which later attaches itself by the pedicle in the 

 typembryonic period. 



From the facts that young individuals of Paleozoic species 

 belonging to such genera as Zygospira, Spirifer, Orthis, 

 Rhynchonella, and Scenidium, have been observed by the 



* Dall in 1870 (Amer. Jour. Conchology) made a clear statement of the 

 characters of Thecidium and of many of its radical points of difference with the 

 Terebratulidae, showing that it was entitled to rank as the type of a distinct 

 family. 



