OF SOUTHER X INDIA. 93 



and shells of these two sub-families arc, however, so perfectly similar, that a separa- 

 tion into two families, as proposed by some authors, would seem unnatural. For 

 instance, the shells of some of the species of Fharella and Nooacidbia are so closely 

 allied, that it is very difficult to distinguish them generically, even as regards the 

 dentition of the hinge, but in Fharella the siplions are united nearly up to the end, 

 as in Solen projier, while in NocacuUiia they are separated for their entire length 

 in a way only to be found in species of TelUna or Scrobiciilaria. The length 

 of the siphons varies in different species of the same genus ; so, for instance, they 

 are in Nov. gaiigetica, usually three times as long as the shell, while in another 

 closely allied species from the neighbourhood of Calcutta, I never observed the siphons 

 longer than about equal to half the length of the shell. In living animals there 

 is a constant current of water observable, entering the lower siphon and exudin<>- 

 from the upper one. The anus terminates in the upper siphon. From time to 

 time the animal of Novae, gangetlca is seen to close both the ends of the siphons 

 and then eject a large quantity of water from the opening just behind the base 

 of the foot. Whether the water previously enters at the same place, or whether 

 it is merely a surplus of that entering through the inhalant siphon, I have not 

 as yet been able to ascertain. 



The shell of all Solenid^ is covered with an epidermis, which encloses both 

 the valves, being open only in front and behind ; it, however, only partially extends 

 beyond the posterior end over the siphons. The valves are long, and, as a rule, 

 quite equal ; the hinge-teeth small, and often thin and laminar, sometimes obsolete, 

 as in the Saxicavid^ ; the ligament external, immediately behind the beaks 

 supported by slightly prominent fulcra ; no cartilage is present, but the ligament 

 is often spongy and soft internally. 



The habits of the so-called razor-fish are too well known to need any further 

 notice ; they possess a world-wide distribution, and live usually in mud or sand 

 between the tide-water marks. A large number are found in tropical and sub- 

 tropical countries in tidal rivers, and some can live for a time even in fresh water, 

 though they soon degenerate in it. Some of the best accounts about this family 

 may be read in Forbes and Hanley's works and other treatises; very valuable 

 information on this subject is also contained in Deshayes' last edition of the 

 Paris Fossils. 



Geologically, the Solenid^ belong to the oldest families of Pelecgpoda ; they 

 are represented in palteozoic rocks, though sparingly. Their number and variety 

 increases very gradually through the succeeding formations, and they have their 

 maximum of development during the present epoch. 



H. and A. Adams, and others, distinguish two sub-families, which are 

 indicated, though by no means very clearly defined, by some differences in the 

 organisation of the animals. They are not accepted by Conrad in his most recent 

 catalogue of the Solenidjs, published in Amer. Journ. of Couch., 1867, vol. Ill, 

 pt. 3, Appendix p. 22, etc. The author quotes 12G recent species belonging to 

 the family. 



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