286 PALAEONTOLOGY OF NEW YORK. 



ments. The foramen, dental plates and teeth are obsolete or obsolescent in the 

 Devonian species* 



In the brachial valve the delthyriuni 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 have been functional 

 at maturity. Muscular arrangement similar to that of Leptana 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 striae; rarely smooth. Shell-substance fibrous, coarsely punctate. 



Type, Lepl(B7ia 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 term ; 

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

 to Stropiiomena (= 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 oblitei-ation of these pai-ts may be due to the excessive secretion of calcareous miitter in the iim- 

 bonal region. Where tliis deixmit is less there remains some evidence of these features, as in 8. profunda of 

 the Niagara, »S'. Becki of the Lower Helderberg, and especially in S. magiiifica of the Oriskany sandstone, 

 one of the largest species of the genus, in which the short dental lamellae converge and unite at the bottom 

 of the rostral cavity, making a sort of pedicle-pit and leaving thi- dclthyrium ojien. See Plate XIII, fig. 28. 



