BRACHIOPODA. 287 



Helderberg, Oriskany and throughout the Devonian, disappearing with the 

 fauna of the Chemung group. 



The Stropheodontas comprise two natural subdivisions based on the contour 

 of the shells alone. The typical group is strongly concavo-convex, and to this 

 belong the majority of the species which can be referred to the genus in its 

 widest scope. Subordinate to this group of convex forms is a smaller division, 

 exemplified by S. nacrea, Hall, of the Corniferous and Hamilton faunas, in which 

 the surface is smooth, often nacreous and with a few squamous growth-lines. 

 The entire substance of the shell is strongly punctate, the epidermal impunctate 

 layer, which in other species preserves its usual thickness, seems here reduced 

 to a mere film. The interior of the brachial valve bears three diverging 

 ridges in front of the muscular area, in this respect resembling Lepttzna rhom- 

 boidalis more than typical Stropheodonta. Closely allied to the species is 

 Strophomena lepis, Bronn, of the Middle Devonian of the Eifel, Belgium and the 

 Asturias.* It may be found convenient to unite these and an unnamed species 

 from the Corniferous limestone under the term Pholtdostrophia. 



The plano-convex species of Stropheodonta are distinguished from the group 

 of S. demissa by more than contour alone. The characters of the deltidium 

 show the same progressive development as in the concavo-convex Strophe- 

 odontas, the earliest species having the delthyrium sometimes open, sometimes 

 partially closed by a convex plate ; while in the Devonian species the deltidium 

 is reduced to a flat, transverse lamina, supported within by the callosity about 

 the cardinal apophyses. In the pedicle-valve are two very strongly pustulose, 

 diverging ridges, bounding the muscular impressions on their lateral margins, 

 while anteriorly these scars are broadly flabelliform and not strongly limited. 

 The central adductors are small, relatively obscure and not divisible, f Should 



* Dr. CEhlbrt associates with. /S". lepis, Bronn, and (S. NaraQomm, de Verneuil (=S. lepis, Bi'onn, teste 

 Kaysbr), tlie finely striated species ^S". clausa, de Verneuil, and (S. Lehlanci, Rouault. This group, he says, 

 forms a passage to the family Productidje, in the rudimentary condition or absence of the foramen, the 

 obsolescence of the teeth and sockets, the arrangement of the muscles, and especially, in the existence of 

 reniform impressions (Ann. des Sciences Geologique, vol. xiv, Art. No. 1, p. 63, 18S7. See pi. iv, fig. 10). 

 Kayser has observed a similar structure in the LeptXna caiidata, Schnur, of the Eifel (Zeitschr. der deutsch. 

 geol. Gesellsch., vol. xxi, p. 628). 



t /S. Calv'mi, Miller, and <S. Cauaoe, Hall and Whitfield, are convex shells with the intei'ior char.actera 

 of this group. Neither their external nor internal features are very positively developed, and these shells 

 are excellent examples of connecting forms. 



