98 



ACROTHELE Linnarsson. 



ACROTHELE AViA. PI. v. Figs, la to/and 2a and b. 

 Acrothele Acrothele avia, n. sp. Nat. Hist. Soc. Bull., vol. iv, p. 202, pi. iii, figs. 



avia. , 



a to h. 



Bull. Nat. Hist. Soc. of New Brunswick, vol. iv, p. 396, pi. xvi, figs. 

 1 a-f and 2 a-b. 



A rather large species with oblately oval valves and a thick horny shell. 



Ventral valve sovnewhat concave in front of the apex. This valve has 

 a triangular, somewhat convex, high area, including a narrow, slightly 

 convex, pseudo deltidium, divided into two equal parts by an obscure 

 central groove. There appears to be a foramina! opening at the slender 

 pointed apex. 



The interior of the ventral valve is marked by a shallow circular pit, 

 on each side of the pedicle opening ; and behind, at the margin, is a 

 shallow triangular pit, resembling the pedicle groove of an O bolus. In 

 front of the pedicle opening is a strong oval tubercle, on each side of which 

 extend the ridges that bound the oval centre of the visceral cavity. Four 

 low vascular ridges extend forward from this oval area to the anterior 

 margin of the valve. The parts of the interior of the shell, above des- 

 cribed, are enclosed by an ox- bow shaped groove, resembling the impres- 

 sion of vascular trunks ; these trunks have about four anterior branches, 

 and each trunk extends nearly to the front of the valve. On the lateral 

 slopes of the shell are three crescentic grooves, which may be accidental 

 and due to pressure. 



The dorsal valve is strongly bent down behind and in front. The umbo 

 is slightly prominent, is appressed, and is close to the posterior margin. 

 The sides of this valve have about a dozen radiating, branching, crenulated 

 ridges, that extend to the margin. The interior of the dorsal valve has 

 a strong median septum, extending to the middle of the valve ; at the end 

 of this ridge is the central muscle. Scars of the latter are seen on each 

 side of the broad end of the median ridge, near its end. On each side 

 of the median ridge at the cardinal margin are two pairs of muscle scars. 

 From the posterior part of the shell several faint radiating ridges extend 

 toward the front margin. 



Sculpture. The surface of the valves is marked by irregular, concen- 

 tric, rounded ridges, that frequently - anastomose ; and the front of the 

 ventral valve and the sides of the dorsal valve have a number of radiating 

 ribs. The sculpture is very variable ; on the central part of the dorsal it 

 shows an irregular network 'of low, rounded ridges ; on the sides of the 



