842 PROCEEDINGS OF THE NATIONAL MUSEUM. VOL.XX. 



Family AECIDJE. 

 BATHYARCA Kobelt. 



Type. Bathyarca pectunculoides (Scacchi). 



Shell oblong, subovate, or rounded, rather thin, usually finely can- 

 cellated, with hairy or scaly epidermis, more or less equilateral, fre- 

 quently slightly inequivalved, with a slight byssal sinus. Byssus very 

 small. Ligameiital area lanceolate, longer and narrower behind the 

 beaks, with a sagittate posterior ligament. Ilinge-margin nearly 

 straight, usually narrow and edentulous in the middle, with a series 

 of small, oblique, striated and crenulated teeth on each end, the distal 

 ones becoming larger and more oblique; those of the posterior series 

 usually longer and more oblique, or divergent, than those in the 

 anterior. 



The animal of 7>. pectunculoides var. grandis, preserved in alcohol, has 

 the margin of the mantle plain without ocelli, with a well-developed 

 muscular septum, posteriorly; the foot large and thick, geniculate, 

 pointed posteriorly, with a strong byssal groove and a slender, solid, 

 byssal stem; two pairs of rather small, long, lanceolate palpi; the rec- 

 tum with a free terminal portion; two pairs of rather large gills, with 

 the posterior end of the stem free for some distance, curved, and 

 tapered to a point, and with the reflected portion of the filament of the 

 same length as the direct; the filaments are very slender, delicate, and 

 soft and but slightly attached to each other. 



This division, which is probably of generic value, includes a number 

 of small and mostly deep-water species which have been variously 

 placed by recent authors. Mr. E. A. Smith puts several of them in 

 Scapliarca with a mark of doubt. Mr. Dall puts two allied species in 

 the Jurassic genus Macrodon^ with which they do not seem to agree 

 very closely, and mentions the affinity of others to Barbatia. 



The last group differs in the stout, rough shell, strongly gaping veu- 

 trally for the large byssus, and in the character of the teeth and liga- 

 ment. ScapTiarca has a thick, strongly ribbed, inequivalved shell, a 

 firm byssus, and continuous, strong, lanceolate ligament. Macrodon 

 has, on the posterior hinge-plate long, divergent lamella?, nearly paral- 

 lel with the dorsal margin. 



We would refer the following species to Bathyarca. B. pectuncu- 



1 The two West Indian species described by Mr. Dall as Macrodon aspenda and 

 M. sagrlnata, should, perhaps, form a separate genus, characterized by the few very 

 oblique, sublamellar, posterior teeth and several smaller, nearly transverse anterior 

 ones. It may be designated as Bentliarca, with Bvntharca aspertila as the type. 



These are closely related to one of the Eocene fossil species (Area adrersidcnlata), 

 which Beshaycs placed in his group of " Cucullaires, ' but later writers (Conrad, 

 isii'l, Fischer, and others) have taken his first species (Jieterodonla) of that group as 

 the type of the genus "Cucullaria," which differs in having the anterior as well as 

 the posterior teeth long and lamolliform ; hence we would associate Tertiary species 

 like Bentharca adversidentata with the living deep-water forms. 



