178 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



and the vertical depression of the head as a whole have caused the lower part of the pre- 

 opercular to project at right angles to the hyomandibular and to assume more than its 

 ordinary share in the work of bracing the quadrate. The preopercular Is stiffened by a 

 sharp boomerang-like ridge on the outer surface, which doubtless also resists the pull of 

 the superficial sheet of the adductor mandibulse. A similar strong ridge on the hyoman- 

 dibular above the preopercular probably has a similar significance. The hyomandibular 

 has the usual two articular heads, connected, as in many isospondyls, by a web of bone. 

 The dilatator operculi fossa, above the hyomandibular, is a narrow groove on the lateral 

 edge of the sphenotic and pterotic. The adductor hyomandibularis was doubtless attached 

 on the concave under surface of the projecting pterotic. The opercular and subopercular 

 are small and leave a wide gap above the opercular, which was doubtless filled by the 

 retractor hyomandibularis muscle. 



The cranial vault is remarkably wide in the top view and shallow dorso-ventrally. 

 The body musculature evidently did not extend forward, there being no posttemporal 

 fossae and no longitudinal crests on top of the stoutly built cranium. The commissural 

 lateral-line canal passes through a slender bony canal lying above the surface of the trans- 

 versely widened parietals and small supraoccipital; the front end of the latter is in contact 

 with the much broadened, coalesced frontals, which, as above noted, are the dominant 

 elements of the skull, extending almost from the occiput to the front part of the elongate 

 snout. The posttemporal is reduced to a slender fork, its third or opisthotic fork being 

 apparently represented by a partly ossified ligament or intermuscular bone (Ridewood, 

 1905^, p. 364). A true though small opisthotic is present on the back of the lateral extension 

 of the exoccipital. This element is absent in many fishes but its presence here may not 

 safely be assumed as a primitive character on account of the unusual specializations of the 

 flattened occiput. 



A very remarkable feature is the presence of a pseudo-occipital condyle, with a convex 

 rather than a concave posterior surface. This condyle, according to Ridewood (1905^, p. 

 363), represents "a portion of a vertebral centrum fused with the basioccipital and lower 

 parts of the exoccipitals." This type of articulation is in wide contrast with the tripartite 

 occipital condyles of typical percomorphs. The convexity of the occipital articulation, 

 notes Ridewood, is not peculiar to Gonorhynchus but is found also in Fistularia and a few 

 other specialized fishes. Very possibly the presence of this comparatively small and weak 

 articulation between the skull and the vertebral column may be connected with the loss 

 of the ordinary extension of the dorsal axial muscles above the occiput. Thus there 

 would be less danger of dorsal displacement of the column and consequent strangulation of 

 the spinal cord. On the other hand, the wide lateral extension of the occipital surface and 

 the flexibility of the functional occipital condyle seem to imply that the column was 

 flexed rather widely in the horizontal plane from the broad occiput as a base. Thus we 

 see that when considered as a natural mechanism this rather peculiar isospondylous type 

 of fish skull is of no little interest. 



Finally, from the taxonomic and phylogenetic side, Ridewood's penetrating analysis 

 (1905^, pp. 363-371) seems to exclude one after another of the long list of isospondyl 

 families from close relationships and to justify his main conclusions, which are as follows: 



"Of the two remaining families which I propose to consider — the Alepocephalidse and 

 Salmonids — the former is to a certain extent specialized in relation with its deep-sea 



