OP SOUTHEEN INDIA. 33 



the difficulty of obtaining a natural arrangement in those species which survived 

 up to the present time is increased. Until, therefore, those connecting links are 

 all properly examined and their relation to the recent species established, we must 

 be satisfied with a provisional grouping. 



The position of the Mactrid^ next to the Myidm and that of the Glaucono- 

 MYiDM as intermediate between the Saxicavibm and the Soljenid^ will probably be 

 found an unusual one, but I think the anatomy of the animals and the general 

 character of the shells fully account for it, as I shall endeavour to prove by 

 giving further details when speaking of these families. I have also placed the 

 Saxicavibje nearer to the Solenid^ as is usually done, not with a desire to 

 diminish the striking relation of Saxicava and Rocellaria ( Gastroch^nib^J, 

 but principally on account of the very great resemblance existing between the 

 Fanope(B, (especially the many fossil species,) and the Fleuromyce and Pholadomyce, 

 and also on account of Cyrtoria very much recalling the form of true Solenibje, 



III. Family,— MTID^. 



The animals of the Myidm generally are unsymmetrical, with a small digitiform 

 foot, and united, more or less distinctly retractile, siphons, prolonged and occasion- 

 ally separated at their ends ; there are two gills on each side, about as long as the 

 posterior half of the body, not prolonged in the siphons ; the labial palpi are pecti- 

 nate and occupy the posterior two-thirds of the anterior half of the body. 



The shell is more or less inequivalve, solid, porcellanous, covered with a thick 

 epidermis, extending over the siphons. The cartilage lies in each valve internally 

 in a special groove, sometimes supported by spoon-shaped processes. Some genera 

 possess a shelly or horny capsule (the ossicle) covering the cartilage internally ; 

 this ossicle is, however, more characteristic for the Anatinidje. 



I accept here the family Myidm inDeshayes' sense, who unites the Corbulid^ 

 and Myid^ of H. and A. Adams into one. They will include the following genera : 

 Simella, Sphenia^ Corhula, Corbulamella, Eucharis, ( JB aster otia), Fleurodesma, Cor- 

 burella, Spheniopsis, Necera, Foromya, Azara, Corbulomya, Cryptomya, Tugonia, 

 Platyodon and My a ; some are divided into one or more sub-genera. 



It is really difficult to make any strict separation of the two families. The animal 

 of My a possesses long united siphons and a deep pallial sinus, while in the other genera 

 the siphons and the sinus are proportionally much shorter. The animal of Tugonia 

 is not known, and its shell is rather thin, as compared with other Myce, but it is of 

 the same type and general form as the rest. Of all the other species the shells, 

 however, agree in their solid structure and in the position of the internal cartilage, 

 the external ligament being either very small, or more often quite obsolete. In 

 order to preserve the one uniform type of shells, it is preferable to treat them in 

 one family, and to distinguish two sub-families, corbulinje d^ndi myinjs. The former 

 will include the genera which usually have a raised cartilage-pit in one and a 

 subtriangular process in the other valve, both being usually closed ; the animals 



I 



