MOLLUSCS. 



269 



trwiscersa the ril)S are about the same in mimliei-, Imt the greater Icngtli of the 

 sliell readily disthigiii^hes it. Both these species occur luuler stones near low-water 

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

 valves are twisted. It is very common in collections. The species of Pectiinculus 

 have a nearly circular outline, and the i-ow 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 freqnentlj' separated undi i i t ninl\, \i ( i i n> k, liut for 

 our purposes they can be retained, as in the 

 older works, as members of the Arcadie. 

 N^ucula embraces small trigonal forms co\ 

 ered with an olive epidermis and a simple 

 jiallial line. Tiie species are mostly inhabi- 

 tants oi the colder water.s of the northern 

 hemisphere. Leda is closely similar to JS^u- 

 cuhi, but has a small pallial sinus, and the 



shell is much longei'. 



- y,.l,l„( luiiatiila 



Yoklia, like the other 

 genera, is boreal in its distrilnition ; in shape 

 and pallial sinus it is much like Lcda., but the 

 siphons are long and slender. Of the sexeral 

 siiecies we need mention but two. Yoldla llm- 

 utida is a very long species, which, according 

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

 an astonishing height, exi'eediiig, in this faculty, 

 the scallop-sliells." Yoldia thixicwfonnis is 

 larger and comparatively shorter. For a long 

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

 was the examination of fish stomachs. More reciently the various dredging exjjeditions 

 lia\e 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 

 s])ecies. 



Of the Teigonid-E, a small and nearly extinct family, but little need lie said. The 

 two valves of the shell are equal, somewhat triangular in outline, ])early inside, and 

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

 hinge teeth are diverging. A somewliat unusual feature is found in the jiosterioi'ly 

 directed umbones. The margins of the mantle 

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

 pal]ii small and pointed. The few existing spe- 

 cies belong to the genus Trigoiiia., and are found 

 oidy in the Australian seas. They are very 

 active and are su])posed to wander about on 

 the sea bottom. 



Far more prominent is the next family, which 

 embraces the fresh-water mussels, the Unioxid^ 

 or Xaiad^ 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 probable that further investi- 

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



tiio rompJanaius, with font .liul 

 siphons extended. 



