OCR Text |
Show 1871.] FRESHWATER SILUROIDS OF INDIA. 717 Pangasius, Pseudeutropius, Callichrous, Wallago, Olyra, Silurus, Plotosus, whilst Chaca is at present doubtful. The general form of the air-vessels is elongated, or heart-shaped, some being posteriorly prolonged. Internally there is an internal septum dividing the organ into two lateral halves, which are sometimes further subdivided by transverse partitions. The central longitudinal septum has an orifice anteriorly affording communication with the two sides; whilst all the lateral chambers communicate with one another on the same, and by means of the anterior one with those on the opposite side. Of those with the air-vessel wholly or partially enclosed in bone, we have Clarias, Saccobranchus, Silundia, Ailia, Ailiichthys, Eutropiichthys, Sisor, Gagata, Hemipimelodus, Bagarius, Pseudecheneis, Glyptosternum, Amblyceps, and Exostoma. The general form of these air-vessels is transverse, with the outer extremities usually dilated and enclosed in bone. Or the air-vessel may be in the form of two rounded lobes enclosed in bone, with a connecting tube, or else the pneumatic ducts coalesce a short distance before they enter the pharynx or upper portion of the alimentary canal. This last division has much in it that is Loach-like in its smaller forms; and Mr. Blyth's observation of Amblyceps, a " Cobitis-hke Siluroid," is still further apparent when we examine its air-vessel. Thus it gradually leads the Siluroids towards the Cobitidina. I do not propose at present to enter further upon the subdivision of the Siluroids, as I hope those who have the opportunity of giving descriptions of the air-vessels of extra-Indian genera will do so, in order to ascertain whether this division is a natural one and applicable to the whole of the family*, as I anticipate it will be found to be. The following notes upon the geographical distribution of these fishes are far from complete; but I deem it advisable to give them as they are, for were I to wait until all that is desirable is effected, I fear the paper would never be finished. Previously to commencing the geographical distribution of the Siluridce, I have some remarks to offer respecting the limits chosen and the terms employed. By India or Hindoostan I understand the whole of the continent under British rule or protection, or independent native states when surrounded by British territory. Burmah includes Arracan and the Tenasserim provinces, whilst the designation Ceylon answers for itself. The East-Indian archipelago has been commonly employed in zoology to express in a general way a large space variously limited or expanded by each fresh author, and is scarcely sufficiently defined when entering upon the distribution of freshwater genera or species offish. I shall therefore employ Wallace's definition of the Malay * Any naturalist who wishes to exchange Tropical forms of Siluroids, personally collected, for those from Hindoostan, will always find m e ready to meet his views. The reason why I wish them personally collected is that the localities may be correctly appended. |