Fishes of the Genus Arges. 531 



belongs, is characterized by tbe presence of a well-developed, 

 freely movable spine, as represented in my figures of 

 Arges homodon and A. Guentheri, and in Boulenger's 

 of ^. Boidengeri. In the three specimens of A. Boulenyeri 

 and the eight of A. Guentheri which I have examined 

 this feature is constant. The next group, comprising 

 A. Eigenmanni^ A. 11 hymperi, and A. Vuillanti, is charac- 

 terized by a weak or moderate, sometimes scarcely distinct, 

 rather elongate adipose fin, with a small but distinct spine 

 constantly present*, more or less projecting in very young 

 specimens and completely imbedded in the adult. This 

 description applies to the single specimen of A. Whymperi, 

 the tliree of A. Vaillanti, and the nineteen of A. Eigenmanni, 

 varying in length from 33 to 100 mm., which I have 

 examined. From these A. orientalis and A. hrachycephalus 

 differ in the absence of the spine, although what appears 

 to be a nodule-like rudiment may rarely be present in the 

 former. I have examined ten specimens of each of the two 

 last-mentioned species. 



I am quite unable to see any discrepancy between Stein- 

 dachner's descriptions of A. sabalo and A. tongifitis and his 

 figures of those species, which are both described and repre- 

 sented as having an elongate and well-developed adipose fin. 

 Such an adipose fin is also characteristic of other species 

 allied to these and represented in the British Museum 

 Collection. 



With regard to the structure of the adipose fin, then, it 

 may be said that, making due allowance for slight differences 

 due to size, individual variation, and state of preservation, 

 there is a remarkable uniformity in members of the same 

 species, whilst between the various members of the genus 

 considerable differences exist, which form a convenient basis 

 for the arrangement of the species. 



I have been able to examine and compare male and female 

 examples in each of the following species : — A. Guentheri, 

 Boulengeri, Eigenmanni, orientalis, hrachycephalus, festa, 

 and peruanus. In none of them can I find the slightest 

 difference between the sexes in the position of the ventral 

 fins, M'hich vary only slightly as to their point of insertion 

 in individuals of the same species, but without regard to sex. 

 Consequently I still attach considerable importance to the 

 position of the ventral fins for the distinction of the species 

 of this genus, whilst the other characters which I have 



« It can always be detected hy ruuniDg the finger-nail along the adipose 

 tin from the tail towards the head. 



