MOLLUSCS. 



269 



transversa the ribs are about the same in number, but the greater length of the 

 shell readily distinguishes it. Both these species occur under stones near low-water 

 mark. Area tortuosa of the Chinese seas is remarkable for the way in which the 

 valves are tw^isted. It is very common in collections. The species of Pectunculus 

 have a nearly circular outline, and the row of comb-like hinge teeth are arranged in a 

 circular arc. As the shell grows, the number of teeth increases by additions to either 

 end of the series. 



The following genera are frequently separated under a family, Nucitlid^, but for 

 our purposes they can be retained, as in the 

 older works, as members of the Arcadae. 

 Nucula embraces small trigonal forms cov- 

 ered with an olive epidermis and a simple 

 pallial line. The species are mostly inhabi- 

 tants of the colder waters of the northern 

 hemisjihere. Leda is closely similar to N-u- 

 cula, but has a small pallial sinus, and the 



shell is 



genera. 



Fig. 2?3. — YoMla limatula. 



much longer. Yoldia, like the other 

 is boreal in its distribution; in shape 

 and pallial sinus it is much like Leda, but the 

 siphons are long and slender. Of the several 

 species we need mention but two. Yoldia lim- 

 atida is a very long species, which, according 

 to Dr. Mighels, has the power of leaping "to 

 ^^ ^^t^^ ^ ^^^^^ ^pi yy^ an astonishing height, exceeding, in this faculty, 

 ^^ „ , ,r ,j ,. ^ • the scallop-shells." Yoldia thracmformis is 



Fig. 284. — Yoldm thracoefonms. ^ . •' 



larger and comparatively shorter. For a long 

 time it was among the rarest of our New England shells, and the only source of supply 

 was the examination of fish stomachs. More recently the various dredging expeditions 

 have found them in large numbers, and the writer remembers that on one occasion 

 the dredge brought up about two bushels of nothing but dead specimens of this 

 species. 



Of the Teigojs^idje, a small and nearly extinct family, but little need be said. The 

 two valves of the shell are equal, somewhat triangular in outline, pearly inside, and 

 are marked by a simple pallial line. The hinge ligament is external, and the few 

 hinge teeth are diverging. A somewhat unusual feature is found in the posteriorly 

 directed umbones. The margins of the mantle 

 are free, the foot is large and long, and the labial 

 palpi small and pointed. The few existing spe- 

 cies belong to the genus Trirjonia, and are found 

 only in the Australian seas. They are very 

 active and are supposed to wander about on 

 the sea bottom. 



Far more prominent is the next family, which 

 embraces the fresh-water mussels, the Unionid^ 

 or N'aiad^ of science. Nearly fifteen hundred 

 nominal species exist in the fresh waters of the world, a large proportion being found 

 in the streams and ponds of the United States. It seems pi-obable that further investi- 

 gations will relegate a large number of these so-called species into synonymy, as many 



Fig. 285. ■ 



- Ujiio complanatus, with foot and 

 siphons extended. 



