CONCIIOLOQY. 12!) 



ScArnri-A, Benson. 



Sliell thin, cquivalvo vciy inecjuil;itoral, liimlir slope keeled, enycrcd with a 

 smooth e]ii(lermis. Auimul liyssifeiovis, and nestles iu cavities in stones in freshwater 

 or lidal streams. 



* rrxNA, B. Teuasserim "River (within the tidewa}'). 



* ," DiiLT.K, W. Blaul'ord. Irrawaddy Delta. 



Sub-family AXIN.^INiE. 

 Foot without a hyssal groove. 

 PECTtTNcrLus, Lamarch. 



Shell ciiuivalvo and eciuilatcral, with prominent ribs. Hinge eurved, teeth 

 pectinate. 



* ,, CASTAXKCs, Lani. (Eceve). 



* ,, I'LAN.vrus, G. and H. Nevill. Andanians. 

 LiMoi'sis coMi'UEssA, G. and II. Nevill. Andamans. 



Fa mi/// Nuculidae. 



Foot compressed, deeply grooved and forming wlien expanded an ovate disk with 

 a sciratcd marsin. Shell pearly. 



Hin-io-linc ann-nlated, with a prominent internal cartilage-pit at the angle. 

 Teeth pectinate. Shell nacreous. 



*NucuLA nmEALis, Hindc. 



,, TnwiiDA, Gould {Jide Mason). 



Faiiiihj Unionidae. 



Slicll cqnivalve. Foot laige, not grooved. The branchial or inhalent region 

 fringed with eirrhi, the exhalent simple. Inhabits fresh water. 



The head-cjuarters of Unio are North Amcnea, from which couiitry npwards 

 of 590 species had been described by Lea up to 1867, and after every deduction has 

 been made for undue multiplication of species. North America will still stand 

 unrivalled not only in the number but in the beauty of the species tenanting its 

 waters. In some species of Unto the form of the male and female shells is different, 

 but this peculiarity is not seen to any marked degree in our Indian Unios beyond 

 a greater tumidity "of the shells in females, the result of the larger space reqtiireil in 

 that sex for their gravid ovaries. The young Unio is matui-ed and hatched within 

 the mantle of its parent. The amount of young matured in a season by a single 

 specimen of Anodonta was estimated at 600,000 by Lea, and may be perhaps put at 

 something less for Unio; still the numbers are enormous, though only a small 

 pcr-centago eventually attains to maturity. Unio is o'ne of those genera which, it has 

 been remarked, seem created for the amusement of species-makers, and it has long 

 seemed to roe desiralilc to group the different races, the result of different elimatal 

 and other surroundings, round a type species, to which they seem nearest allied, or 

 whence derived ; thereby breaking up this overgrown geuus into manageable groups, 

 each group in reality representing one species, with its associated or derivative races 

 or sub-species. By following this plan we have in Burma six species, embracing 

 over 20 so-called species or races already ascertained, and some probably yet to be dis- 

 covered. 



What some naturalists would seem to understand by species is not easy to 

 understand. C. Wyville Thomson, after sne(;ring, in his ])rclaee to "The Depths 

 of the Sea" (p. 11), at the Darwinian doctrine of descent by modification of one 

 species from another as "only a hypothesis," goes on to add: "During the whole 

 period of recorded human observation not one single instance of the change of one 

 species into another has been detected ; and singular to say, in successive geological 

 formations, although new species are constantly appearing, aud there is abundant 



e 



