Thomson . — Brachiopod Genera . 



395 



primitive form it is very short, being confined to the posterior part of the 

 hinge-trough, and is also very low, in shape being a pyramid with an edge 

 facing forwards and a hollowed side behind. In more advanced types the 

 process gains both in length, occupying more and more of the hinge- 



Fig. 2. — Interiors of dorsal valves of Pachymagas and Neothyris, showing evolution of 

 cardinal process, a. P. parki (Hutton), Kakanui greensands. b. P. parki (Hutton). 

 Hutchinson's Quarry greensands. c. P. hultoni sp. now (see appendix), Wharekuri 

 greensands. d. N. ovalis (Hutton), Shakespeare Cliff, Wanganui. e. N. ovali* 

 (Hutton), Castlecliff, Wanganui. /. N. lenticularis (Desh.), Recent (after Davidson). 



trough, and in height, projecting above the socket-ridges, while the 

 hollow behind is now confined to its base, and is bounded by two 

 lateral ridges. In the most advanced types met with in New Zealand 

 species of Pachymagas there is a slight posterior median ridge in the 



