82 ANATOMY OF VERTEBRATES. 



which project forward, and are connected at the middle line 

 with a like series from the opposite side of the head ; they may 

 be regarded as partial dismemberments of the great pectorals ; 

 and in Rhinoptera Braziliensis their supporting cartilage is 

 directly continued from that of the pectoral fins, though it is 

 closely attached to the fore part of the head. These form what 

 Miillcr has termed 'cranial fins;' but the parts more properly 

 meriting that name are the opercular and braiichiostegal appen- 

 dages of the tympanic and hyoidean arches. 



28. Skull of Protopteri. - -Thus far we have seen that the base 

 of the skull is first formed by the anterior prolongation of the 

 notochord and the expansion therefrom of its capsule ; and that the 

 cranial cavity results from the extension of the outer layer of that 

 membrane over the anterior end of the nervous axis. We saw 

 next the superaddition of special capsules for the organs of sense ; 

 and the cartilaginous tissue developed in the notochordal sheath 

 at the base and sides of the cranium, according to a pattern 

 common to the lowest and to the embryos of the higher vertebrata. 

 We saw the cartilaginous tissue acquiring a firmer texture, hard- 

 ened by superficial osseous grains, or tesserae, mounting higher 

 upon the lateral and upper walls of the cranium, and at length 

 entirely defending it : and we then also recognised the maxillary, 

 mandibular, and hyoidean arches, established in a firm cartilaginous 

 material, and on a recognisable ichthyic type. 



We have now to trace the course and the forms under which 

 the osseous material is superadded to, or substituted for, the 

 primitive cartilaginous material of the skull ; and the remarkable 

 Lepidosiren, whose organisation was first made known as in the 

 generic form called Protopterus, 1 offers a transitional step, in the 

 shape and structure of its skull, between the gristly and the bony 

 cold-blooded vertebrates. 



In the Lepidosiren, ossification of the cranial end of the noto- 

 chord extends along the under and lateral part of its sheath, 

 backward to beneath the atlas, fig. 41, i, the posterior slightly 

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

 the neurapophyses of the atlas, fig. 66, ?i, the bases of which 

 expand and meet above the notochord and below the spinal 

 canal. Ossification of the notochordal sheath commencing at 

 its under part, ib. b, ascends upon the sides of the notochord as 

 it advances forward, and encloses it above, where it supports the 

 medulla oblongata, and the lateral bony plates (neurapophyses) 



1 XXXIII. 



