Raymond — Common Devonian Brachiojoods. 283 



slightly above, the level of the posteriors. Between the scars, 

 on the median line, is a low, short septum which is hardly ele- 

 vated above the surface of the shell in young specimens, but 

 becomes prominent in adults. On either side of it is a low 

 ridge extending back nearly to the base of the cardinal process. 

 In the adult the portion of these ridges in front of the muscle 

 scars becomes high and incurved, and may function as a sup- 

 port for the brachia. These ridges are short, and their anterior 

 ends are not half-way to the front of the valve. They appear 

 to be homologous with similar ridges in Chonetes scitulus, 

 which are certainly connected with the brachia. The struc- 

 ture of the muscle scars and ridges in this species should be 

 compared with that in the dorsal valve of S. concava. In 

 neither case do the ridges in front of the muscle scars func- 

 tion as the attachment for muscles, as has been suggested by 

 many writers. 



Hinge Structure. — A few of the smaller specimens show a 

 short exsert pedicle tube. In adult and senile stages the 

 pedicle opening is pushed back onto the beak, and is very 

 minute. 



Stropheodonta perplana Conrad. 



(Plate XIII, Row 1.) 



Pal. N. Y., iv, 1867, pp. 92, 98, pi. 11, fig. 22 ; pi. 12, figs. 13-15. 



Protegulum. — The protegulum of this species is nearly cir- 

 cular, biconvex, with arcuate hinge. It measures '10 X'10 m,n 

 on one specimen, and •12X"12 mm on another. 



Nepionic Stage. — The shell in the nepionic stage is convex 

 in both valves, nearly as long as wide, and both valves are 

 smooth. On the dorsal valve is a fold which extends nearly to 

 the front of the shell (figure 1). 



Changes dtcring Development. — Like l 



S. inmquistriata^ this shell becomes 

 strongly alate in the neanic stages and 

 the hinge width remains the greatest 

 width throughout life. (See specimens 

 4, 7, 10, and 14 of the series, for exam- „ 



i c l\ ' \ a £i. xi • • Figure 1. — Stropheo- 



ples of this.) After the nepionic stage, donta perplana CoE T rad . 

 the dorsal valve becomes first flat and dorsal valve, showing shell 

 then slightly concave. The shell re- in nepionic stage, the fold, 

 mains nearly flat throughout all stages. < **^ e origin of the striae ' 



Mtiscle Scars. — In this species there 

 is no change in the position of the muscle impressions during 

 life. The adductor scars in the ventral valve are divided by a 

 diagonal line into anterior and posterior elements, a fact not 

 shown in calcified specimens. 



