SELENELLA 243 



Pedicle valve with its greatest depth a little posterior to the middle, 

 subcarinate along the median region from the beak nearly to the front 

 margin, on either side of the rather narrowly rounded median region the 

 surface slopes to the extreme lateral extremities with a gently concave 

 curvature producing a compression of the valve in that direction, near the 

 postero-lateral margins the surface is abruptly curved towards the oppo- 

 site valve, becoming inflected to the cardinal extremities, from the highest 

 point to the front margin the surface is gently convex and to the beak 

 a little less gently convex; mesial sinus absent, the subcarinate median 

 portion of the valve simulating a mesial fold; the beak acutely pointed, 

 strongly protuberant beyond that of the opposite valve, nearly erect, the 

 foramen not clearly shown in the type but apparently small and encroach- 

 ing upon the umbonal region, the characters of the delthyrium and 

 pseudodeltidium hidden in the specimen. Internally a pair of dental 

 lamellae extend anteriorly from the beak for about one-fifth the length of 

 the valve with a very narrow angle of divergence. 



Brachial valve much flattened, less convex than the pedicle, the greatest 

 depth posterior to the middle, the surface very gently convex to the 

 antero-lateral and anterior margins, curving more abruptly to the postero- 

 lateral margins, especially as it approaches the margin; mesial fold or 

 sinus absent; the beak moderately incurved, passing beneath the delthy- 

 rium of the opposite valve. 



Shell structure minutely punctate. 



Remarks. In general outline this shell resembles Centronella alveolata 

 Hall from the Onondaga limestone of New York, but it is a smaller and 

 much thinner shell with the pedicle valve less arcuate and without a 

 median sinus in the brachial valve. The internal characters of this 

 species, aside from the dental lamellre of the pedicle valve, are unknown, 

 though it can be affirmed that no median septum is present in the brachial 

 valve, that valve being so exfoliated in the type specimen that it would 

 be clearly shown if present. The species is quite certainly not a true 

 Centronella, because in all cases where the form of the brachidium is 

 known among these shells, important modifications in the Centronella type 

 of loop have become fixed before the beginning of Mississippian time but 

 without a knowledge of the characters of the brachidium, it is unwise 

 to propose a new generic division, and the species is allowed to rest in 

 Centronella with a query. 



Horizon. Burlington white chert. 



Genus SELENELLA Hall 



Description. Shell small, terebratuliform, with biconvex, smooth valves, 

 subovate in outline. Internally the brachidium is centronelliform, but 



