Report of the State Geologist, 



529 



CHARDIA, Mki.KIM.I.NA, M.\.,\s M A. . \ si i I. \ ;m.| TeBEBRATI 



successively. The larval conditions of the terminal members of 

 both series are similar t<> the adult condition of the simplest 

 members of the family, Cistella ami Gwynia. 



Thus similar resultants have ensued in this family by progres- 

 sive growth and resorption of tin.' brachial supports through 

 quite dissimilar lines of development.* 



From the study of young stages in a Carboniferous species, 

 Didasma twrgida, a terebratuloid with short, recurved loop, and 

 in the Silurian species, Zygospira recurvi- 

 rostra, a spire-bearing shell with introverted 

 cones, it has been demonstrated! that in 

 the earliest observed conditions of both, 

 the brachidium consists simply of the two 

 lateral or descending lamella?, united in 

 front, forming an expanded triangular plate 

 like that in Centronella and Rensse- 



LAERIA. 



, Fig. 186 — Dielasma elonga- 



Eesorption of the anterior portion of this t«. /, foramen; jjugai pro- 

 plate gradually produces the ascending cesses: d,dentalpl,tes;m.mus- 

 r ° J r ™ ~ & cu iar impression.— (Davidson.) 



and transverse lamellse of Dike asm a and 



the jugum of Zygospira ; parts which are thus seen to be 

 alike in origin and function. With Dielasma the process here 

 ceases, but in Zygospira there is a continuation of the lamellae 

 carried forward from the ante-lateral angles of the loop, which, 

 with growth takes on the spiral form of the mature brachia. 

 The inference from this evidence is that the Centronella-Ioop is 

 of the simplest type; anything more elementary would repre- 

 sent the condition of the clescrete crural processes in Khyn- 



chonella. The recurved loop of the 

 terebratelloids is a more advanced 

 condition of this organ, while the spiral 

 brachidium, attained so abundantly and 

 with such diversity, during Paleozoic 

 and early Mesozoic time, represents its 



Fig 



Rhynchotreta cuneata 

 showing the crural processes. 



most elaborate development. 



* These interesting facts have been established by the investigations of Friele, CEhlert, 

 Fischer and C. E. Bekcher. 



IBeecher and Schuchert; Proceedings of the Biological Society of Washington, vol 

 VIU, 1893. 



1891. 67 81 



