78 LECTURE IV. 



and giving the peculiar form to tlie skull of certain Sharks, thence 

 called " Hammer-headed " {ZygcBna). In the Eagle-ray {JSIyliohates) 

 a cartilage is attached to the anterior prolonged angle of the great 

 pectoral fin, and connects it with the fore-part of the cranial (inter- 

 nasal) cartilage; it supports a number of branched and jointed car- 

 tilaginous rays, which project forwards, 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 Brazilie?isis 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 Miiller has termed 

 "cranial fins;" but the parts more properly meriting that name are 

 the opercular and branchiostegal appendages of the tympanic and 

 hyoidean arches. 



Having traced in the examples of cartilaginous fishes selected for 

 demonstration, the progressive steps by which the typical features of 

 the ichthyic skull are modelled, as by the hand of the sculptor, in the 

 yielding gristle, we have next to consider them with their leading 

 varieties, as they are permanently wrought out in hard bone. 



We saw that the base of the skull was first formed by the antei'ior 

 prolongation of the gelatinous chorda dorsalis, and that the cranial 

 cavity resulted from the extension of the membrane from the fibrous 

 sheath of the gelatinous chorda over the anterior end of the nervous 

 axis. We saAV next the superaddition of special capsules for the 

 organs of sense ; and then the cartilaginous tissue developed from 

 the basis cranii, according to a pattern common to the lowest forms 

 of the class, and to the embryos of the higher forms which the Cy- 

 clostomes permanently represent. We saw the cartilaginous tissue 

 acquiring a firmer texture, hardened by superficial osseous grains, or 

 tesserce, mounting higher upon the lateral and upper walls of the 

 cranium, and at length entirely defending it : and we then also i-e- 

 cognised the maxillary, mandibular, and hyoidean arches, established 

 in a firm cartilaginous material, and on a recognisable ichthyic type. 



We have next 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 in osseous fishes ; and the remark- 

 able transitional genus Lepidosiren, whose organisation I first made 

 known under the name of Protopterus (xxxin.), offers the most 

 natural and instructive passage in the shape and structure of its 

 skull, between the gristly and the bony fishes. 



In the Lepidosiren ossification of the cranial end of the chorda 

 dorsalis extends along the under and lateral part of its sheath, back- 

 wards to beneath the atlas and axis i^fig. 27. i), the posterior slightly 



