112 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



gill-bearing arch) is more elongated and slender than the hyoid arch and is like- 

 wise divisible into two pieces, an anterior ceratobranchial and a posterior, 

 slightly longer, epibranchial. Between the median ventral portions of the 

 hyoid and third arch is a triangular copula representing a basibranchial piece. 

 The fourth and fifth gill arches are short curved rods of cartilage on each side, 

 composed of epibranchials. At the anterior end of the epibranchial of the fourth 

 arch is a small ceratobranchial. In the median ventral line is a slender bone, 

 the second basibranchial. It will be seen that the gill arches of Necturus are 

 reduced both in number and as regards the pieces of which they are composed 

 as compared with the gill arches of the elasmobranchs. For the typical condi- 

 tion of the gill arches in elasmobranchs see Figure 33, page 100, and compare 

 with the condition in Necturus. 



F. THE SKULL OF THE ALLIGATOR, A TYPICAL MODERN SKULL 



The skull of the alligator is a nearly completely ossified skull in which the 

 various components of which the skull is composed are so closely knit into a 

 single structure as to be inseparable. The skull of the alligator is probably as 

 typical and generalized a skull as is to be found among living land vertebrates 

 and has further the advantage of large size. The student must learn the bones 

 of the skull and jaw of the alligator and be able to distinguish the cartilage from 

 the membrane bones. Figures of the alligator skull will be found in R, pages 

 252, 254, 256, 261; P and H, page 342; CNA, Vol. VIII, page 458; K, page 104. 



i. General regions and cavities of the skull. Obtain a skull and study its 

 structure. It is composed of a number of separate bones closely jointed to each 

 other along somewhat jagged lines known as sutures. The various components 

 of which we learned the skull is constructed are here morphologically indistin- 

 guishable from each other but will be pointed out later. The bones of the roof 

 of the skull are pitted and roughened. At the anterior end of the dorsal surface 

 are the two nasal openings or anterior nares. Posterior to the middle of the 

 roof are the two large orbits, the largest openings in the roof of the skull. Pos- 

 terior to each orbit is the temporal region in which are several holes, known as 

 fossae or -vacuities. The development of these temporal fossae is characteristic 

 of reptiles and has occurred during their evolution, since the early reptiles, as 

 in Figure 36.6, possessed completely roofed skulls. (See further on this point, 

 R, pp. 285-87, and Fig. 44, p. 176.) The alligator skull has two pairs of these 

 openings, a dorsal pair the supratemporal fossae, on the dorsal side of the pos- 

 terior end of the skull and a lateral pair the infra-temporal fossae, just posterior 

 to the orbits from which each is separated by a rod of bone, the postorbital bar. 

 Lateral to each supratemporal fossa is a projecting ledge of bone, the supra- 

 temporal arcade. Underneath this overhanging ledge is an opening, the external 

 auditory meatus, which leads into the cavity of the middle ear or tympanic cavity. 

 Numerous canals and passages will be found entering the tympanic cavity, and 



