THE SKULL OF FISHES. 



expanded end of this ossified part supporting, as in the Squatina, the 

 ncLirapophyses of the atlas {fig. 28. ii), the bases of which expand 

 and meet above that end of the ossified chorda and below the spinal 



^^p^^^rrmTTTTTTTTWr 



27 



BV 



Skeleton of Lcpidosircn anncctens. 



canal. Ossification of the fibrous sheath of the chorda, commencing 

 posteriorly at its under part (ib. b), ascends upon the sides as it 

 advances forwards, and incloses it above, where it supports the me- 

 dulla oblongata, and the lateral bony plates (neurapophyses) called 

 ex-occipitals {ib. 2) ; leaving behind a wide oblique 

 concavity lodging the anterior unossified end of the 

 ' chorda,' which does not extend further upon the 

 'basis cranii.' The ex-occipitals {Jig. 21, 28. 2, 2), 

 Atlas and expand as they ascend and converge to meet above 



occipital vertebra, , n )i-ii a 



Lcpidosircn. the ' lorameu magnum which they complete. A 

 small mass of cartilage connects their upper ends with each other, and 

 with the overhanging backward projecting point of the fronto- 

 occipital spine {ib. a). This cartilaginous mass answers to the base 

 of the supra-occipital in better ossified fishes : a similar cartilage 

 connects the ex-occipitals with the occipital spine in the Tetrodon. 



We clearly perceive in the Lepidosiren that ossification, advancing 

 on the common cartilaginous mould of the plagiostomovis skull, has 

 marked out the posterior cranial vertebra, and not only its neura- 

 pophyses but also its centrum ; the neural spine being left in a less com- 

 pletely ossified state than in the vertebraa of the trunk. The occipital 

 pleurapophyses (scapulte, fig. 27. 51 ) are much more developed, and 

 appear as two strong, bony, styliform appendages, articulated by a 

 synovial capsule and joint, one on each side, to the persistent carti- 

 laginous base of the neurapophyses (ex-occipitals), and partly to the 

 centrum or basi-occipital. To the lower and less expanded ends of 

 the pleurapophyses are attached the extremities of the hasmapo- 

 physes (coracoids, _/?y. 27. 52) ; and thus is completed the ha3mal arch 

 of the occipital vertebra, here unusually developed in relation to its 

 ofl&ce of protecting the heart and pericardium : the haemapophyses or 

 coracoids belong to the same category of vertebral elements as the 

 sternal ribs which protect the heart in higher Vertebrata. The costal 

 or liaimal arch of the occipital vertebra of the Lepidosiren supports 

 an appendage {fig. 27. 57), projecting outwards and backwards like 



