80 



LECTURE IV. 



the simple diverging appendages to the abdominal pleurapophyses 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 solitary I'ay) of the pectoral fin ; it 

 is the key to the homology of the anterior or upper limbs of the 

 higher Yertebrata, showing them to be appendages of the hsemal arch 

 (usually called scapular) of the occipital or posterior cranial vertebra. 



In the second (parietal) and third (frontal) cranial vertebrae, ossi- 

 fication 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, rej^resenting at its posterior end 

 the body of the atlas and the basi-occipital, expands as it advances 

 along the base of the skull in the situation of the sphenoids, consti- 

 tuting 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 basi-cranial bone 

 may be defined by a slight transverse depression, where also termi- 

 nates a median longitudinal groove, traversing the under part of the 

 thus defined occipital portion of the bone ; and indicating, like the 

 corresponding membi'anous fissure in Cestracion, the primitive place 

 of the cranial end of the chorda. The expanded sides, originally 

 arches of the cartilaginous portion, bend down to abut against the 

 bases of the pterygoid plates. In this expansion of the basi-sphenoid 

 the Lepidosiren resembles the Plagiostomes and also the Batrachian 

 Reptiles. 



Two ridges rise from the upper surface of the occipito-sphenoidal 

 plate, near its outer margin, and support the cartilaginous lateral 

 walls of the cranium. The cranial cavity is defended above by a 

 longitudinal bony roof {^fig. 27. 3, 7, ii), nearly co-extensive with the 

 bony floor beneath ; the roof commences behind by the spine or point 

 which overhangs the ex-occipitals, gradually expands as it advances, 

 resting upon the cartilaginous walls of the cranium, is then suddenly 

 contracted, and is united anteriorly by fibrous ligament to the ascend- 

 ing process of the palato-maxillary arch, and to the base of the nasal 

 plate. A strong sharp crest or spine rises from above the whole of 

 the middle line of the cranial roof-bone, which may be regarded as 

 representing the mid-frontal, the parietal, and supra-occipital bones, 

 or, in more general terms, the neural spines of the three cranial 

 vertebrae : but this supra-cranial bone not only covers the medulla 



