188 PROF. T. H. HUXLEY ON MENOBRANCHUS. [Mar. 17, 



fig. I ), presents the form of a pentagon, with a deeply excavated base, 

 and with the angle opposite the base truncated. This truncated 

 angle corresponds with the ends of the premaxillary bones ; the 

 lateral angles are occupied by the extremities of the suspensoria, or 

 peduncles to which the mandible is attached ; the posterior angles 

 answer to the epiotic processes of the skull of an osseous fish. As 

 these project beyond the level of the occipital foramen, they give rise 

 to the excavated contour of the base of the pentagon. The occipital 

 condyles lie one on each side of and below the occipital foramen ; 

 and their slightly convex free surfaces look inwards and backwards. 

 In a side view (Plate XXIX. fig. 1), the skull is seen to be much 

 flattened from above downwards. The suspensorium is inclined 

 downwards and forwards at an acute angle with the cranio-facial 

 axis. The ramus of the mandible is proportionally stout and thick, 

 and the hyobranchial apparatus, though almost wholly cartilaginous, 

 is massive and large relatively to the skull. 



The skull consists of a cartilaginous framework, in and upon 

 which certain ossifications have been developed. The former is 

 what is commonly termed the "primordial cranium;" but, as it is 

 preceded by a membrauous structure, it would be better termed the 

 " chondrocranium," while the bony skull may be called the " osteo- 

 cranium." 



The osteocranium consists of the following bones : — 



1. The exoccipitals (E.O). — These lie on each side of the 

 occipital foramen, and bear the condyles. They do not come into 

 contact either above or below; but the small space left between them 

 is covered by the parietal bones in the former region, and by the 

 parasphenoid in the latter. 



2. The epiotics (Ep.O). — I term these interesting ossifications 

 " epiotic " for brevity's sake. In reality they represent not only the 

 epiotic but the opisthotic ossifications of other Vertebrata. They 

 are conical caps of bone, which are separated by narrow cartilaginous 

 interspaces from the pro-otics (Pr.O) in front, but closely unite with 

 the exoccipitals behind and below. As usual, they shelter the pos- 

 terior part of the organ of hearing; and the fenestra ova/is, with its 

 stapes (St), is situated in the unossified interspace between the an- 

 terior and inferior (or opisthotic) part of the bone and the pro-otic. 



3. The pro-otic ossifications (Pr.O) occupy their ordinary place 

 in the front part of the capsule of the organ of hearing, and are 

 covered above by the parietals (Pa.), and externally by the squa- 

 mosals (Sq.). 



4. The parasphenoid (P.Sph) is a very wide and thin bone, 

 which extends from the lower margin of the occipital foramen, 

 posteriorly, to a point beyond the middle of the length of the vomers, 

 anteriorly. It underlies the exoccipitals, the epiotics, and the pro- 



Bten Segmente der eigentlichen Schedeleapsel nicht ossificirt," and that they have 

 an abortive cartilaginous antorbital process. He also mentions that the liga- 

 ment which extends from the prsemaxilla to the suspensorium contains particles 

 q{ cartilage feingesprengte Knovpel). I have not noticed these in my specimen. 



