ox THE AXATOMY OP FISHES. 
259 
the remarkable structural modifications involved in the forward dislocation of the 
anterior vertebi’m and the associated portions of the Weberian mechanism into the 
cranial cavity, and in the combination of a partially ossified air-bladder with external 
osseous capsules, Hypophfhahnus is unique among all S. ahnormales which have so 
far been described, and represents the extreme of a series of modifications of which 
Loricaria and its allies exhibit the initial stages.”' 
From the facts above-mentioned it may be concluded : (1) That the Hypostoma- 
tinm and Hypophthahninfe exhibit the most extreme modifications which the air- 
bladder, Weberian mechanism and the correlated skeletal elements undergo in the 
Siluridse. (2) That the modifications in question have to some extent taken place 
in the same direction, so that the two groups possess certain features in common, and, 
notably, the firm sutural union, or even partial anchylosis of the anterior vertebrae 
with the skull, and also, with one exception [Acantliiciis)^ in the mode in which the 
osseous capsules for the air-bladder are formed ; in the partial or complete atrophy 
of the bony roof of the cavum sinus imparls ; and in certain other features which 
the two groups share with the less aberrant Siluridae abnormales. (3) That the 
exceptional features so characteristic of certain genera, viz., the formation of complete 
bony capsules for the air-bladder by the actual ossification of its own walls in 
Acanthiciis, and the peculiar relation of both to the first pair of ribs ; and the singular 
dislocation of the anterior vertebrae in IIypop)litliahniis, are sufficient to prove that 
very striking variations may occur within the limits of the two groups. 
A careful investigation of the remaining genera of the Hypostomatinae may bring 
to light other forms, some of which may prove to be even more, and others less, 
aberrant than those already known. 
We have had no opportunity of dissecting the Indian genera, Sisor, Exostoma, and 
Pseudecheneis, which are the only Hypostomatinae that are not restricted to South 
America. Som.e doubt has been thrown on the coiTectness of their presumed relation- 
ship to this group, but the mode of formation of the osseous capsules for the air- 
bladder, and the extent of coalescence among the anterior vertebrae, should furnish 
decisive evidence on this point. It is unfortunate, therefore, that the only evidence 
available through Day’s descriptions is confined to the bare fact that the air-bladder 
consists of two lateral sacs enclosed within bony capsules, no information being 
given about the special features so necessary for comparison with undoubted 
Hypostomatinae. 
The singular air-bladder of Ageniosus militaris (Muller, loc. cit.) bears a remark- 
able resemblance to that of such Cyprlnidae as Cohitis and Botia. Not only is the 
* As Ramsay Wright (44) points out, Uypophthalmns occupies a somcwliat isolated position ainoiif^ 
Siluroids. GtxTHER (15) forms for the genus a distinct sub-familj. Cope (7) regards the genus as 
the type of one of the three families into wliich he divides tlie Xematognatlii (= Siluridai) ; and, 
finally, Gill (13) reserves for it one of his eleven families of Xematognatlii. 
2 L 2 
