C. E. Beecher — Development of the Brachiopoda. 141 



within the cephalic segment. Notwithstanding these dif- 

 ferences, so many parts are functional equivalents, that their 

 growth and development may be discussed and interpreted in 

 the same terms. . 



Before passing to later stages of growth which become 

 more and more divergent from a common simple type, some 

 points previously omitted, relating to Thecidium (Lacazella), 

 Lingula (Glottidia), and Discinisca, should be here noted. As 

 Lacazella is a form in which the ventral valve in the nealogic 

 and ephebolic stages is cemented to foreign objects by cal- 

 careous fixation, it bears about the same relation to other 

 brachiopods that Ostrea bears to Avicula, among the pelecy- 

 pods, and a corresponding early absence, or modification, of 

 many features present in adult individuals should be looked 

 for. From what is known of the geological history of Theci- 

 dium, and if the interpretations of its phylogeny by the writer 

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

 condition of fixation as early as the Upper Silurian. Theci- 

 dium is apparently not a terebratuloid genus. Its structural 

 affinities are evidently with the strophomenoids, especially 

 such forms as Plectambonites, Leptaenisca, etc. Briefly the 

 reasons for this statement are (a) the presence of a deltidium 

 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 place in the cavity of the ventral beak, bearing the 

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

 cardinal processes in the dorsal valve.* The first character is 

 of prime importance, because all the strophomenoids 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 (Glottidia) described by Brooks, 4 and the 

 Discinisca by Miiller, 20 both representing the phylembryonic 

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

 mentary pedicles. Terebratulina becomes attached or rests on 

 the caudal segment during the cephalula stage (Morse), while 

 at the end of this period in Cistella (Kovalevski and Shipley), 

 there is an active, swimming, ciliated organism, which later 

 attaches itself by the pedicle in the typembryonic period. 



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

 of Thecidium and of many of its radical points of difference with the Terebra- 

 tulidse, showing that it was entitled to rank as the type of a distinct family. 



