122 



THE ELASMOBRANCH FISHES 



Fig. 121. Teeth of Heptanchus indicus. 

 donald and Barron.) 



(From Mac- 



mouth and nasal pits in He/ptanchus are not connected by oronasal grooves 

 characteristic of some of the Elasmobranchs. The buccal cavity proper is 

 large and spacious. Its floor is lifted up by the basihyal cartilage, forming a 

 skeleton for the so-called tongue. The mucous membrane lining the mouth is 

 provided both dorsally and ventrally with numerous stomodeal denticles (see 

 p. 24, fig. 27c) which, as we have seen in a study of the integument, are modi- 

 fications of placoid scales. In the region just within the crescent of teeth 

 the lining of the mouth is thrown into heavy folds, which lie over the concen- 

 tric rows of tooth buds. 



The teeth of Heptanchus (see fig. 48, facing p. 44) consist of a heavier lower 

 series and a cuspidate upper series. The first tooth in the lower row of Hep- 

 tanchus maculatus, like that 

 for H. indicus (fig. 121), is 

 unpaired and without a me- 

 dian cusp. On either side of 

 it are seven teeth, the most 

 posterior of which is cusp- 

 less and is followed by sev- 

 eral rows of smaller flat 

 nodules not shown in figure 

 48. The first of the large 

 teeth is provided with a se- 

 ries of three conules on the median margin and usually six larger conules on 

 the lateral margin. Other lower teeth including the sixth, though differing in 

 size, are essentiallj^ like the first paired tooth. Unlike Heptanchus indicus (fig. 

 121) there is usually no unpaired upper tooth in Heptanchus maculatus. The 

 first paired tooth above bears a long fang which is directed downward; at its 

 sides are small basal denticles like those in Heptanchus indicus. The second 

 tooth and the ones following, although larger, differ from the first only in that 

 they possess median conules and outer cusps, several of which are present on 

 each tooth. 



Pharynx and Associated Structures 



The pharyngeal part of the tract is wide from side to side and depressed or 

 flattened dorsoventrally. Through its ventrolateral walls are perforations, the 

 internal branchial openings by means of which the respiratory current reaches 

 the gill pockets (see fig. 142, facing p. 148). These openings are of interest 

 in a consideration of the respiratory system. The lining of the pharynx, like 

 that of the buccal cavity, is provided with denticles, but these on the roof in 

 the posterior part are confined to a narrow strip just above the internal bran- 

 chial clefts. The pharynx narrows toward the region of the oesophagus, which 

 is closed except during the passage of food. By this closure of the oesophagus 

 the buccal cavity and pharynx form a relatively large room. 



In connection with the pharynx are the thymus and thyroid glands. The 

 thymus in Heptanchus, figure 122, lies dorsal to the first six gill clefts and 

 takes the form of bunches of grapes. Van Wijhe has made the remarkable dis- 



