338 Prof. Owen's Description of the Lepidosiren annectens. 



tremities, and are more elongated, forming the first of the caudal series of 

 vascular arches already described. 



The pelvic arch is represented by a single piece of cartilage of a crucial form * ; 

 the transverse pieces curve slightly upwards, and we may suppose them to re- 

 present the iliac elements of the os innominatum : the articular surface for the 

 basis of the posterior extremity is near the anterior part of the cartilage. This 

 support of the rudimental ventral fin consists of a single-jointed soft rayf, si- 

 milar to that of the anterior extremity, but thicker ; about forty joints may be 

 counted in this ray, in many of the larger of which there were ossific deposits. 



In reviewing the principal characters above noticed of the skeleton of the 

 Lepidosiren, we obtain good evidence of its ichthyic nature. If, indeed, the 

 species had been known only by its skeleton, no one could have hesitated in 

 referring it to the class of Fishes ; but in that class it would have offered a 

 most singular and interesting combination of the cartilaginous and osseous 

 types. 



The central elements of the vertebral column, — the basis of the skeleton, — 

 exhibit a persistence of its primitive embryonic condition, such as has hitherto 

 been witnessed only in the Sturgeon and Cyclostomous fishes ; but the supe- 

 rior arches and the spinous appendages, instead of retaining the cartilaginous 

 state, are converted into the tough elastic fibrous texture characteristic of the 

 skeleton of fishes. The cranium in like manner presents an extremely novel 

 combination of the cartilaginous and bony states both as regards its partial 

 ossification and the condition of the ossified parts. It is only in the higher 

 cartilaginous fishes, e.g., that the maxillary, palatine and pterygoid bones are 

 blended together to form the simple superior dentigerous arch or upper jaw. 

 The composition of the lower jaw corresponds with that which characterises 

 most of the osseous fishes, and is more simple than in the Amphibia. The 

 confluence of the cranial vertebrae reminds one of the condition of the skull in 

 the Siren : but no vestige of a preopercular bone is present in any of the Peren- 

 nibranchiates. The " sphenoideum basilare" as it exists in the Sturgeon is here 

 seen in its fully ossified state. As the basis of the vertebral column presents a 

 condition analogous to that which characterises the early embryonic periods of 



* Tab. XXIII. fig. 4, y. t lb. z. 



