186 Scientific Proceedings, Royal Dublin Society. 



mediately below these, and separated from them by a slit-like 

 depression, are a pair of sub-cjlindrical articular processes, which 

 are united very firmly to the ends of those of the exoccipital bone. 

 They are hollow, and contain a core of cartilage,^ which is conti- 

 nued into the exoccipital. The lower surfaces of these processes 

 articulate with the bevelled upper border of the basioccipital ; and 

 below this the joint resembles an ordinary articulation between two 

 vertebrae. On the side of the body, just below the posterior zyga- 

 pophyses, is a small pit, for the attachment of a ligament passing 

 to the supraclavicle. 



The second vertebra presents nothing remarkable, except the 

 shortness of body and absence of parapophyses common to the first 

 three vertebrse. 



The third vertebra (PL YIII., fig. 21) has a shorter neural 

 spine than either the second or fourth. On the side of the body 

 between the anterior and posterior zygapophyses is a deep pit, with 

 which the head of the first rib movably articulates. 



Caudal Vertebrce. — The eighth caudal may be taken as a 

 typical member of the series. The centrum is more perfectly 

 circular than that of an abdominal vertebra, and, owing to the 

 absence of parapophyses, the sides appear more flattened. They 

 are beautifully sculptured, the anterior and posterior edges being 

 raised, and a ridge running along the middle, above and below 

 which are large oval depressions. The neural spine is longer and 

 more slender'and curved than that of an abdominal vertebra, and 

 with a greater backward inclination. The zygapophyses are not 

 so well developed. The haemal arch and spine are almost exactly 

 like the neural, and provided with anterior and posterior zygapo- 

 physes, which are, however, more imperfect than the superior. 



1 On removing the bone from the surface of this cartilage, it is seen to be attached 

 by its broader end to the atlas. It passes from thence into the exoccipital bone, where ' 

 it tapers to a point. If.it be removed entire, and surrounded by a thin shell of bone, 

 and a longitudinal section made, it is seen to be composed, entirely of hyaline cartilage, 

 the cartilage passing into the osseous tissue of the atlas behind, and into that of the 

 exoccipital in front. It presents the appearance of an 'extosteal ossification extending 

 from the atlas, and a similar extension from the exoccipital bone around a continuous 

 rod of cartilage. This seems to imply a continuity of tissue between the parachordal 

 cartilages and the first vertebra, such as occurs in the sturgeon (accipenser) ; but this 

 is so much opposed to our present knowledge of bony fishes, that it could not be 

 accepted, unless corroborated, by embryological research. 



