58 PROCEEDIXGS OF THE XATIOXAL MUSEUM. vol.37. 



viously expressed the opinion that the spiral of Helicoprion was 

 composed of the symphysial teeth of the upper jaw, the spiral 

 having been pushed outside of the mouth and carried above the 

 snout. 



Eastman based his conclusions on the fossil called Campodus^ 

 which he shows was composed of the symphysial teeth of probably 

 the lower jaw. This roAv of teeth would corresj^ond to the median 

 row of lower teeth mlleterodontus phih'ppi. the Port Jackson shark. If 

 in this shark the outer and older teeth should, instead of dropping 

 off, cohere with the younger teeth, there would be formed at least 

 two-thirds of a coil, a structure that would resemble that of 

 Campodxs. If. then, the teeth .should become strongly compressed 

 the mass would resemble considerably that object that we call here 

 Toxoprion. Further coiling would result in a series like Lisso prion. 



However, when Ave come to homologizing Edestus, Lisso prion, and 

 HeJicoprion with the teeth of Campodus and Heterodontus diffi- 

 culties are encountered. In the case of the two latter genera, the 

 difficult}' is to determine what disposition to make of such large 

 spirals. If in Heterodontus the symphysial teeth should cohere with 

 one another, a spiral of several coils might eventually be formed; 

 but unless there were some especial arrangement developed, the spiral 

 could be completed only by a pushing of the older end of it through 

 the skin and into the flesh and cartilage of the jaw. This would not 

 contribute to the comfort of the animal or the strength of the jaw, 

 however much it might aid our efforts at homologizing. It would 

 be necessary, too, to conjecture a shark with a lower jaw of tremen- 

 dous proportions to accommodate a spiral like that of Helicoprion., 

 the diameter of which is sometimes as much as 260 mm. If it be 

 said that the spiral projected far enough beyond the jaw to escape 

 burial in the tissues, it may be objected that it would have been in 

 a position to be troublesome to the animal and exposed to injuries. 

 The slender and bent dental mass of Toxopnon., too, would have hung 

 down in a position dangerous to its existence. 



A strong objection to placing any of these fossils in the mouth of a 

 shark is to be found in the fact that none of them show an}' indica- 

 tions of wear. The species of Edestus, described above, present no 

 attrition of the enamel or of the most delicate denticles or crenations. 

 Dr. A. S. Woodward, in speaking of Helicoprion ° has sought to 

 escape this objection by supposing that the rows of teeth were so 

 far apart that they did not rub against one another. Nevertheless, 

 constant contact with the food taken into the mouth must have pro- 

 duced some wear. 



It seems certain that the general conclusions of Karpinsky regard- 

 ing Edestus and Helicoprion must be accepted, namely: (1) These 



° Geol. Magazine, dec. 4, 1900, vol. 7, p. 33. 



