IN THE PABAGTJAYAN LEPIDOSIEEN, ETC. 363 



the ramus ophthalmicus of the Fifth nerve. The roots of the first two spinal nerves 

 are transmitted through two separate foramina in the exoccipital bone. 



The dorsal extremity of the pectoral girdle is connected with the occipital region of 

 the skull by a stout fibrous band, in which is embedded what Parker [32] describes as 

 " a thin, subfalcate, subcutaneous bone " (I. c. p. 21), and which he designates the " post- 

 temporal." The bone is relatively much smaller than in Ceratodus, and has apparently 

 lost its direct articulation both with the skull and with the supraclavicle. 



C. Lepidosiren. 



In the main the skull of Lepidosiren closely resembles that of Protopterus. The 

 most important of its structural peculiarities may be briefly summarized as follows : — 



The chondrocranial portion of the skull exhibits further indications of atrophy, 

 especially in the occipito-periotic region. The cartilaginous roof, which in Protopterus 

 extends as far forward as the anterior limit of the auditory capsules, has receded 

 somewhat in Lepidosiren and is now restricted to the supraoccipital region. The 

 exoccipitals are invested neither externally nor internally by cartilage, and hence the 

 complete occipital ring of cartilage in Protopterus becomes divided in Lepidosiren into 

 a dorsal and a basal plate, which are only connected through their continuity anteriorly 

 with the periotic capsules. The trabecular cartilages are considerably reduced in 

 thickness and in height, and are now represented by relatively slender, laterally- 

 compressed bands, slightly increasing in vertical extent as they become continuous 

 with the periotic and suspensorial cartilages, but only to a very slight extent do they 

 become visible from the interior of the cranial cavity. The styliform process of the 

 mesethnoid cartilage is also shorter than in Protopterus. On the other hand, the 

 antorbital cartilages are somewhat better developed, and, in addition to supporting the 

 inferior and hinder margin of the upper labial folds, are prolonged into anteriorly- 

 directed pi*ocesses. 



The suspensorial cartilage is essentially similar in structure to that of Protopterus, 

 except for the emargination of that part of its posterior margin which is covered 

 externally by the squamosal. Undoubtedly its most interesting feature is the rotation 

 backward of its articular extremity to a greater extent than in any other Dipnoid, and 

 hence its forward inclination is less, and the angle which it makes with the fore part 

 of the basicranial axis greater, than in either Ceratodus or Protopterus. The effect of 

 this recession of the condylar extremity of the suspensorium on the palato-pterygoid 

 bone is that the latter is not only of greater relative length, but deviates more from 

 the vertical, and describes a segment of a larger circle in passing forward to its 

 symphysial termination, than in either of the other two genera. 



The two exoccipital bones are distinct, but nevertheless closely approximated both 

 dorsally and ventrally. 



