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sphenoid and backwards to beneath the atlas and axis, the posterior slightly expanded end of 

 this ossified part supporting, as in the Squatina, the neurapophyses of the atlas, the bases of 

 which expand and meet above that end of the ossified chorda, and below the spinal canal. 

 Ossification of the fibrous sheath of the chorda, commencing posteriorly at its under part, 

 ascends upon the sides as it advances forwards, and incloses it above, where it supports the 

 medulla oblongata, and the lateral bony plates (neurapophyses) called ' exoccipitals,' leaving 

 behind a wide oblique concavity lodging the anterior unossified end of the ' chorda,' which 

 does not extend further upon the basis cranii. The exoccipitals expand as they ascend, and 

 converge to meet above the foramen magnum, which they complete. A small mass of carti- 

 lage connects their upper ends together, and with the overhanging, backward projecting point 

 of the occipito-frontal spine. This cartilaginous mass answers to the base of the superocci- 

 pital in better ossified Fishes : a similar cartilage connects the exoccipitals with the occipital 

 spine in the Tetrodon. It is plain, in the Lepidosiren, that ossification, advancing on the com- 

 mon cartilaginous mould of the plagiostomous skull, has marked out the posterior cranial 

 vertebra, and not only its neurapophyses, but also its centrum ; the neural spine being left in 

 a less completely ossified state than in the vertebrse of the trunk. The occipital pleurapo- 

 physes ('scapulae') are much more developed, and appear as two strong, bony, stvliform 

 appendages, articulated by a synovial capsule and joint, one on each side, to the persistent 

 cartilaginous base of the neurapophyses (exoccipitals) and partly to the centrum or basi- 

 occipital. 



To the lower and less expanded ends of these pleurapophyses are attached the extremities 

 of the hsemapophyses (' coracoids ') ; and thus is completed the haemal arch of the occipital 

 vertebra ; here greatly expanded, as in other fishes, in relation to its office of protecting the 

 heart and pericardium : the hsemapophyses or coracoids belong to the same category of ver- 

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

 or lucnial arch of the occipital vertebra of the Lepidosiren supports an appendage projecting 

 outwards and backwards, like the simple diverging appendages to the abdominal pleurapo- 

 physes of better ossified Fishes, and like the costal appendages in the thorax of Birds ; but it 

 is here cartilaginous, and consists of many segments. It forms, in fact, the rudiment (a soli- 

 tary ray) of the pectoral fin : it is the key to the general homology of the anterior or upper 

 limbs of the higher Vertebrata ; showing them to be appendages of the haemal arch (usually 

 called ' scapular '), of the occipital or posterior cranial vertebra. The suspension of the sca- 

 pular arch directly to the skull is an evidence of the piscine nature of the Lepidosiren. 



In the second (parietal) and third (frontal) cranial vertebrse, ossification extends along the 

 basal and along the spinal elements, but not into the neural or lateral elements ; these remain 

 cartilaginous in continuation with the cartilage surrounding the large capsule of the internal 

 ear. The basal ossification, representing at its posterior end the body of the atlas, then the 

 basioccipital, expands as it advances along the base of the skull in the situation of the sphe- 

 noids, constituting the floor of the cerebral chamber, supporting the medulla oblongata, the 

 hypophysis, the crura and lobes of the cerebrum, and terminating a little in advance of the 

 olfactory lobes by a broad transverse margin, bounding a triangular space left between it and 

 the converging palatine arches, which space is filled by cartilage, representing the vomer. The 

 occipital part of this basicratiial bone may be defined by a slight transverse depression, where 

 also terminates a median longitudinal groove, traversing the under part of the thus defined 



