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, Leptcenisca, etc. Briefly 

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

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

 the arms (the internal calcification or spiculization is confuied 

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

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

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

 (b) 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 larvse as described by Kovalevski 

 and Lacaze-Duthiers. 



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

 the Biscinisca by Miillerj^" both representing the phylem- 

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

 rudimentary pedicles. Terehratulina 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 



* Ball in 1870 {Amer. Jour. Concholor/i/) made a clear statement of the 

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

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

 family. 



