ANATOMY OF VERTEBRATES. 83 



called exoccipitals, ib. 2 ; leaving behind a wide oblique conca- 

 vity lodging the anterior unossified end of the 

 notochord, which does not extend further upon the 

 basis cranii. The exoccipitals, ib. 2, 2, expand as 

 they ascend and converge to meet above the ' fora- 

 men magnum' which they complete. A small mass ^* 



J, ., 1 • T • 1 1 Atlas and 



ot cartilage connects their iipi)er ends with each occiritai vertebra, 

 other, and with the overhanging backwardly pro- 

 jecting point of the frontoccipital spine, ib. 3. This cartila- 

 ginous mass answers to the base of the superoccipital in better 

 ossified fishes : a similar cartilage connects the exoccipitals with 

 the occipital spine in the Tctrodon. 



We clearly perceive in the Lepidosiren that ossification, ad- 

 vancing on the common cartilaginous mould of the piscine skull, 

 has marked out the neurapoph3^ses and centrum of the posterior 

 cranial vertebra. The occipital pleurapophyses, called ' scapidie,' 

 fig. 41, 51, appear as strong, bony, styliform appendages, articu- 

 lated by a synovial cajisule and joint, one on each side, to the ex- 

 aiid basi-occipitals. To the pleurapophyses are attached the upper 

 extremities of the ha3mapophyses (coracoids, fig. 41, 52) Avhich 

 unite together below, and thus complete the hajmal arch of the 

 occijiital vertebra, here unusually developed in relation to its 

 office of protecting the heart and pericardium. The coracoids 

 belong to the same category of vertebral elements as the sternal 

 ribs which protect the heart in higher Vertebrata. The lucmal 

 arch of the occipital vertebra of the Lepidosiren supports a filiform 

 appendage, ib. 57 ; it is the key to the homology of the anterior 

 or upper limbs of the higher Vertebrata. 



In the second (parietal) and third (frontal) cranial vertcl3ra3, 

 ossification extends along the basal and along the spinal elements, 

 but not into the neurapophysial or lateral elements ; these remain 

 cartilaginous in continuation with the cartilage surrounding 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 

 sphenoids, constituting the floor of the cerebral chamber, sup- 

 porting 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 the persistent ' vomerine ' cartilage. The sides 

 of the basicranial plate bend down to abut against the bases of 

 the pterygoid plates. In this expansion of the Ijasisphenoid the 



G 2 



