SKELETON OF EEGALECUS AEGENTEITS. 27 



8. The Hip-girdle and Pelvic Fin. (Plate IV. fig. 6, and Plate V. fig. 19.) 

 The os innominatum. — The hip-girdle consists of two distinct innominate bones * each 

 of which has the form of a triangle with an elongated base and greatly produced 

 posterior angle. From its inner face arises a vertical ridge (fig. 19, sy), which passes 

 inwards and is united to its fellow of the opposite side by ligament, forming an anterior 

 symphysis ; a posterior symphysis is formed by a similar ligamentous union of the 

 whole of the opposed postero-superior borders (sy 1 ) of the innominate bones. On the 

 ventral border, at about the junction of its anterior and middle third, the os innominatum 

 is greatly thickened and forms a large saddle-shaped surface or an acetabular facet for 

 the pelvic ray. The ridges with which the bone is marked all radiate from this facet. 



In the natural position of the parts, the antero-superior portion of the hip-girdle is 

 embraced and hidden by the shoulder-girdle (fig. 6). 



The pelvic (so-called ventral) Jin (fig. 6 and 19, pr.f), consists of a single slender ray, 

 articulating by a thickened, saddle-shaped, proximal end with the acetabular facet. In 

 the specimen both pelvic rays were broken, but the more perfect of the two was 3 feet 

 1 inch long, and tapered so nearly to a point that it is improbable that more than a 

 few inches were lost. The proximal end of the ray is about ^ inch thick. 



Summary. 

 The following are the most important osteological peculiarities of Begalecus 

 argenteus : — 



1. Owing to the large size of the eyes as compared with that of the cranium, the 

 orbit is enlarged by the formation of a subcranial crest (PL IV. figs. 7, 10, 11) formed 

 by a downward prolongation of the basis cranii and having a triradiate horizontal section 

 (p. 7, also vide infra §§ 5, 9, and 17). 



2. There is no supraoccipital crest, nor any epiotic or parotic processes (p. 7, figs. 7, 8, 

 and 11). 



3. A large part of the cranium remains cartilaginous, there being a well-developed 

 and very thick tegmen cranii (figs. 8, 11, and 12, t.cr) quite uncovered by bone, and a 

 large prenasal rostrum (p.n) ; a considerable part of the auditory capsule also remains 

 unossified (pp. 8 and 9). 



4. The perichonclium lining the cranium is so thick as quite to alter the shape of 

 the cavity (p. 9, figs. 11 and 12). 



5. The basioccipital (figs. 7-13, b.o) does not enter into the formation of the foramen 

 magnum, being covered above by the united exoccipitals (e.o) ; it furnishes a medio- 

 ventral facet to the occipital condyle (o.c 2 ), and is produced downwards into a median 

 vertical plate, which forms the postero-superior portion of the subcranial crest (p. 9). 



6. The exoccipitals (e.o) unite with one another below, thus forming the ventral as well 



1 Davidoff (Morph. Jahrb. vol. vi. p. 433) considers the so-called os innominatum or pelvic bone of Teleostei 

 to be really the basipter3'gium ; but his views require confirmation. 



e2 



