BRACHIOPODA. 345 



SpiRiFEBiD^, the Atbyrid^, the Meristidm, and all genera with everted brachidia 

 are related to the Atrypib^ only through their early ancestral forms. 



The Lower Silurian faunas have furnished no evidence of species with 

 everted spirals, and this hiatus in our knowledge forbids any satisfactory deduc- 

 tions as to the source or derivation of these forms. It is true in a general 

 sense that the eversion of the spirals is accompanied by a convexity of both 

 valves, just as the inverted spirals of the Atrypivm are associated with valves 

 of notably unequal depth. Still, among the latter, Glassia possesses biconvex 

 valves, while of the former the group composed of C(elospira, Anoplotheca, 

 KoNiNCKiNA and Amphicuna, is characterized by convexo-plane or convexo- 

 concave valves. In this group also the apices of the spirals are not directed 

 toward the lateral commissures of the valves, but toward the lateral slopes of 

 the pedicle-valve, such a form and direction being a necessary outcome of the 

 contracted interior space. From present evidence it would seem probable that 

 among the early Silurian species will be found some form whose spiral ribbon 

 deviates outwardly from the vertical plane to the same degree as it inclines 

 inwardly in Ctclospira and Peotozyga. Indeed, in Cyclospira bisulcata itself, 

 the spiral sometimes lies so nearly in the vertical plane that the inward inclina- 

 tion of the apices is not always positive. Only some such form of the earliest 

 faunas could have been the progenitor of the everted spirals. 



In the Atrypid^^ possibilities of a variation in the form of the jugum were 

 much restricted ; in the other groups of the spire-bearers these were very great, 

 and resulted in the production of a wonderful series of modifications whose 

 relations it is not necessary to rehearse here. The extreme range of these 

 modifications is seen in the simple termination of the jugum in Wiiitfieldella, 

 Rhynchospira, etc. ; the bifurcate extremity in Meristina, Eumetria and Retzia, 

 their terminal branches in Kayseria, Diplospirella, etc., finally becoming co- 

 extensive with the lamellsB of the primary spirals and thus forming a second 

 pair of spiral cones. This complication of the brachidium is effected only late 

 in the history of the various groups producing them. Koninckina and Amphi- 

 cuna are double-spiraled convexo-concave shells, which are the post-palceozoic 

 and final representatives of Anoplotheca and Coelospira. Pexidella and Diplo- 



