286 PALEONTOLOGY OF NEW YORK. 



ments. 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 

 apophyses are strongly arched into the uinbonal cavity of the opposite valve, 

 their surfaces of attachment being sometimes 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 liave been functional 

 at maturity. Muscular arrangement similar to that of Leptczna rhomboidalis and 

 Rafinesquina alternata, 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 apo- 

 physes. The median septum often becomes elevated into a high crest at the 

 center of the valve. Over the pallial region the interior of both valves is 

 strongly papillose. External surface covered with radiating, sometimes fas- 

 ciculate strise; rarely smooth. Shell-substance fibrous, coarsely punctate. 



Type, LepliZna demissa, Conrad. Hamilton group. 



Observations. The distinctive characters of this genus are clearly evident, 

 but notwithstanding its importance, both zoologically and geologically, it has 

 never been accorded general recognition except among American writers. Mr. 

 Billings would not admit its validity ; Dr. Davidson barely noticed the terra ; 

 Professor Kayser and tlie German writers generally continue to refer its species 

 to Strophomena (= Strophomena, Lept^na, Rafinesquina, etc.) ; Dr. CEhlert 

 has adopted it, making it a sub-genus of Strophomena, Rafinesque (de Blain- 

 ville), while he proposes a new genus, Douvillina, which is essentially synony- 

 mous. The genus Stropheodonta is a large one, being represented in American 

 faunas by not less than fifty species, and it is emphatically characteristic of the 

 Devonian. It makes its first appearance in the Clinton group, having a sparse 

 representation in the Niagara, but becomes more abundant in the Lower 



* The obliteration of these parts may be due to the excessive secretion of calcaieous matter in the uni- 

 l)onal region. Where this deimsit is less there remains some evidence of these features, as in S. profunda of 

 the Niagara, )S'. lievki of the Lower Helderbei-g, and esijecially in xS. magnifica of the Oriskany sandstone, 

 one of the largest species of the genus, in which the short dental lamelliE convei-geand unite at the bottom 

 of the rostral cavity, making a soi-t of pediclo-jiit and leaving thc> dcllliyrium open. See Plate XIII, tig. 28. 



