202 BULLETIN OF THE NATURAL HISTORY SOCIETY. 



Pal^obolus Bretonensis. PI. II., figs. 2a to i. 



Oblately orbicular. Valves evenly rounded from the centre, 

 except that the borders are flattened at the sides and front. Both 

 dorsal and ventral valves somewhat pointed at the umbo, which is 

 depressed in both valves. Interior of the ventral. — This has a broad 

 hinge area and a triangular pedicle groove. The visceral cavity has 

 two pairs of diverging ridges, which mark the advance of the lateral 

 muscles during the growth of the shell. Between the outer and the 

 inner pair originate the vascular trunks, which in going forward throw 

 off branches at an acute angle. Interior of the dorsal valve. — This 

 valve has a broad, transversely striated hinge area. The visceral 

 cavity is traversed by two pair 5 of diverging ridges, more widely 

 divergent than those of the ventral valve ; there is also a strong- 

 median septum along the middle of the valve. The central group of 

 muscle scars are about a fifth from the front of the valve. 



Sculpture. — The whole outer surface, except close to the umbo, is 

 ornamented with sharp concentric ridges which occasionally anasto- 

 mose ; these ridges have fine, faintly marked, radiating stria? on their 

 posterior slopes, and are obscurely crenulated along their crests. 



Size. — Length 15 mm., width 17 mm. Dorsal valve somewhat 

 shorter than the ventral. 



The Obolus nearest this in age is 0.(1) major, of the Upper 

 Etcheminian in New Brunswick ; that species, however, is larger, and 

 does not have the concentric ridges, on the only example of it which 

 is known. None of the European Oboli have the sharp ridges which 

 mark the surface of this species, and they all belong higher in the 

 geological scale. Obolus (Mickwilzia) monilifera, Linrs., has a flat- 

 tened dorsal valve, and thus differs from this species. 



Acrothele avia, n sp. Pi. III.; figs. In to h. 



Calcareo-corneous. A rather large species with oblately oval 

 valves, thick, horny, shell substance within, and a thin, calcareous 

 crust without. 



Outwardly the ventral valve differs little in shape from the ordin- 

 ary types of Acrothele of the Protolenus and Paradoxides beds ; 

 internally, however, it is characterized by a single in place of the 

 paired pits in front of the foramina! opening, and by strong vascular 

 trunks and branches. 



