can be common inheritance. The question whether the roof 

 and cheek of the actinopterygian were already largely de- 

 fined at the time of origin of this line must remain unan- 

 swered, although comparisons suggest that the roof was and 

 the cheek was not. The fact that among crossopterygians 

 and dipnoans there are several cheek patterns supports this 

 view. In most respects, the general pattern of actinoptery- 

 gian and crossopterygian are the same, particularly in terms 

 of the visceral skeleton. The greatest area of disagreement 

 is in the dermal cover of the mandible. To be sure, some 

 actinopterygians also deviate from the basic roof pattern, 

 just as the dipnoan and actinistian or some individuals 

 within any species, deviate. 



The number of ossifications in the otic region of the 

 actinopterygian endocranium is larger than in the choanate 

 or amphibian. The appearance of a supraoccipital in the 

 teleost suggests a parallelism to that of the higher tetrapods. 

 The metapterygoid ossification of the palatoquadrate ap- 

 pears to be the homolog of the choanate epipterygoid. 



In summary, one can say that the actinopterygian and 

 choanate are much alike; from this it follows that the com- 

 mon ancestor was not far removed from either or from the 

 amphibian. The Osteichthyes as a natural group appears to 

 be legitimate but might include the tetrapods. The problem 

 now becomes one of looking for features distinguishing bony 

 fishes from the other kinds of jawed fishes, or to search for 

 the roots of the Osteichthyes among the other gnathostomes. 



Chondrichthyes 



The chondrichthians, acanthodians, and arthrodires are 

 sometimes associated together on the basis of the similarity 

 of their chondrocrania. This association of forms, which at 

 first glance seem to be so diverse, tests the criteria used in 

 the preceding discussions, both in terms of how these are 

 used and their intrinsic value. 



As presently conceived, the cartilaginous fishes (Chon- 

 drichthyes) are subdivided into two subclasses: the Elasmo- 

 branchii, and the Holocephali. The first of these is a large 

 array of species which can be separated into the sharks, rays, 

 and torpedos. Of these only the shark will be considered. 



Sharks The head skeleton of the shark is cartilaginous 

 throughout with some calcification in the basis cranii. This 

 condition is typical of the group, which extends back to the 

 Upper Devonian. Teeth, which may be those of sharks, come 

 from earlier rock strata. 



The Dogfish Shark, Sgualus acanlhias, is most commonly 

 dissected. It will be compared with other sharks such as 

 Heptanchus, Chlamydoselachus, and Hexanchus. 



The endocranium of Squalus (Figure 5-16) is much like 

 that of some osteichthians. It differs in the flaring orbital 

 margins, the presence of an optic pedicel, the weak develop- 

 ment of the olfactory capsules, the large precerebral fossa, 

 and the capsular canal passing from the brain cavity 



below the nasal capsule. The basis cranii shows perichondral 

 calcification from the foramen magnum to the connection 

 with the preorbital plate. This calcification occurs at both 

 the inner and outer surfaces and extends laterally to the 

 edges of the basal plate and ventral margin or the orbit. 



The palatoquadrates are joined anteriorly by con- 

 nective tissue; lateral to this union, each has a tall 

 orbital process with a medially and posteriorly facing 

 articular surface. The whole articular area is U-shaped and 

 fits up around the braincase in front of the basal processes. 

 The lateral aspect of the basal process connects through a 

 thick pad of connective tissue with the medial surface of the 

 palatoquadrate somewhat behind the vertical of the ascend- 

 ing process. 



The palatoquadrate extends out and back to the angle 

 of the mouth, where it articulates with the mandible. There 

 is an upward flaring adductor process above the articular 

 condyle. 



The palatoquadrate of Heptanchus articulates with the 

 cranium in two places: at the basal process and also with 

 the postorbital process. The postorbital articular part of the 

 palatoquadrate is identified as the otic process, or quadrate 

 process. Two areas of articulation identify the amphistylic 

 (both pillar) style of jaw suspension as opposed to the 

 hyostylic (hyoid pillar supported) type of Sgualus. The 

 former is observed in the most primitive sharks, the latter in 

 the more advanced forms. 



The lower jaw is made up of the joined Meckel's carti- 

 lages and is short and massive. Associated with the jaws are 

 labial cartilages: two with the palatoquadrate and one with 

 the mandible. In Heptanchus all three appear to form a 

 single Y-shaped piece. 



The "hyomandibula" articulates with the posterior end 

 of the cranium below the lateral head vein. In the other fishes 

 the articulation is above the vein. The hyomandibula, or 

 epihyal, of Squalus is a short irregular piece, distally bound 

 ligamentously to the palatoquadrate and articulating with 

 the stout ceratohyal. The latter in turn articulates with 

 a slightly curved transverse plate, the copula, representing 

 the fused basihyals. It is generally assumed that the hypo 

 elements are missing in this group, although a vestigal one 

 is found in the first branchial arch of Scyllium and in several 

 arches of different rays. The assumption is based on the 

 proposition that hypohyals continue the line of the cerato 

 element forward and medially, while the basi elements tend 

 to be single midline pieces extending posteriorly behind the 

 tip of the lower arm of the arch. 



There are five branchial arches in Squalus (seven in 

 Heptanchus and Chlamydoselachus), each (from top down) with 

 pharyngobranchial, epibranchial, ceratobranchial, and 

 basibranchial components. The fifth arch is the exception, 

 lacking a separate pharyngobranchial because of fusion with 

 that element of the fourth arch. The pharyngobranchials 

 extend posteriorly, and only the anterior part of the first one 

 is attached to the cranium. The main attachment of these 

 elements is to the spinal column. 



GNATHOSTOME FISHES • 119 



