596 Forty-fifth Report on the State Museum. 



Stropheodonta, Hall. ' 1852. 

 (Plate 14, Figs. 1-6.) 



Shells normally concavo-convex. Cardinal area common to the 

 two valves, that of the brachial valve narrower or linear; smooth 

 or finely striated longitudinally, sometimes showing through the 

 outer lamina? of the shell, strong, parallel, transverse bars, which 

 project beyond the cardinal margin as short processes, and articu- 

 late with a corresponding series of pits in the opposite valve. 

 The earliest species have these denticulations developed only for 

 a short distance on each side of the center, while in the latter 

 faunas they extend the entire length of the hinge-line. In the 

 type-species the delthyrium is usually completely closed by the 

 extension of the shelly lamina? of the area, and on the inside by 

 the formation of a callosity between the apophyses of the cardi- 

 nal process. Sometimes a low, narrow, convex deltidium is 

 present, and in the early species the delthyrium is frequently 

 open. 



From the normal position of the dental lamella? there extend 

 two diverging, sometimes nearly vertical ridges of variable 

 strength bounding the post-lateral portion of the muscular area, 

 which is not limited by a riclge in front. The entire muscular 

 area consists of two broad flabelliform diductor scars, inclosing 

 an oval adductor which is distinctly divided into anterior and 

 posterior elements. The foramen, dental plates and teeth are 

 obsolete or obsolescent in the Devonian species. 



In the brachial valve the delthyrium is also usually closed, the 

 cardinal apopt^ses are strongly arched into the i mbonal cavity 

 of the opposite valve, their surfaces of attachment being some- 

 times nearly parallel to the plane of the cardinal area of the 

 brachial valve, and often extending beyond it. Small crural 

 plates are always present, though they could not have been 

 functional at maturity. Muscular arrangement similar to that of 

 Rqfinesquina altemata, the posterior scars being more elongate 

 the anterior pair usually less defined, and all the scars frequently 

 obscured. The anterior muscular fulcra are sometimes developed 

 into very prominent elongate apophyses. The median septum 

 often becomes elevated into a high crest at the center of the 

 valve. Over the pallia] region the interior of both valves is 

 strongly papillose. Externa] surface covered with radiating, 



148 



