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 Leptcma 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 Pholidostrophia. 
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. (Ehlert associates with S. lepis, Bronn, and /S. Narajoana, de Verneuil ( =S. lepis, Bronn, teste 
Kayser), the finely striated species S. clausa, de Verneuil, and /S'. Leblanci, Rouault. This group, he says, 
forms a passage to the family P roductidjf., 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, 1887. See pi. iv, fig. 10). 
Kayser has observed a similar structure in the LeptcBna caudata, Schnur, of the Eifel (Zeitschr. der deutsch. 
geol. Gesellsch., vol. xxi, p. 628). 
t /S’. Calvini, Miller, and /S’. Canace, Hall and Whitfield, are convex shells with the interior characters 
of this group. Neither their external nor internal features are very positively developed, and these shells 
are excellent examples of connecting forms. 
