February 1, 1901.] 



KNOWLEDGE 



•Id 



mcut by the ordinary soft-scaled fishes so abundant in 

 the present seas. As the majority of these old mail-clad 

 fishes, as well as a large proportion of the ancient sharks 

 were provided with crushing teeth, it is a fair inference 

 that their food consisted largely of shell-lish and crusta- 

 ceans with a certain proportion of their own mail-clad 

 relatives. When, however, the swift^swimming soft- 

 scaled fishes came to the fore, they would naturally 

 offer a more tempting and nourishing diet to such 

 sharks and other prcdaceous members of their own class 

 as were swift enough in their movements to make them 

 their prey. And consequently the old millstone-jawed 

 sharks would tend to more or less completely disappear. 

 On the other hand, the skates and rays, which are for 

 the most part slow-moving creatures, flapping sluggishly 

 along on the sea-bottom by means of their fan-like tins, 

 would be quite unable to capture the modern type of 

 swift-swimming fish. And they have thus had to con- 

 tent themselves with the old-fashioned diet of shell- 

 fish and crabs, in consequence of which a large propor- 

 tion of them have retained the dental millstones which 

 have been so steadily going out of fa.shion among their 

 more advanced relatives. Not that these rays and 

 skates have by .any meaus been content with the kind 

 of molar machinei-y that did duty for their forefathers, 

 since some of them, together with their Tertiary ances- 

 tors, have developed what appears to be an absolutely 

 perfect type of living mill, far superior to that which 

 served the purpose of their predecessors. And it must 

 always be remembered that these beautiful living mill- 

 stones and cylinders (which are some of the most exquisite 

 bony structures to be met with in the whole animal 

 kingdom) excel their artificial substitutes in that they 

 never wear out ; being renewed either by the develop- 

 ment of new teeth on the inner or hinder aspect of the 

 cylinder, or by vertical successors replacing the indi- 

 vidual teeth from below or above. 



And now that the dental millstones of the rays have 

 been mentioned, it will afford a convenient starting- 

 point for a brief survey of some of the most remarkable 

 types of structure presented by these curious organs. 



The teeth of rays always form a pavcracnt-like 



Fig. 4. — Upper leetL of BcakeU Bay {Rhynchobalia). 



Flu. .5. — Part of the I'ahitc of Lepidoius, m\ Eiianiel-srijUil Fi-li 

 I'rom t]ic AVt-altlcii. 



structure, of which the comj^oneiit elements are arranged 

 in straight longitudinal rows, although (Fig. 4) they may 

 likewise show a quiucunxial mode of arrangement. The 

 individual teeth are not replaced by vertical successors; 

 but, being in the form of a half-cylinder, as those in 

 front become worn down, the whole series is pushed 

 forwards, and new teeth are developed on the hinder 

 margin of the cylinder. The supreme development of 



a dental structure ;idapted for crushing in this group 

 occurs in the family of the eagle-rays {M i//iohiitl'hr), 

 in which the millstone (Fig. 1) of each jaw forms a 

 iierfect semi-cylinder or plate, made up of flat-crowned 

 prismatic teeth united at their edges, often so as to 

 constitute a mosaic-like pavement. No piece of modern 

 machinery can be better adapted for crushing hard sub- 

 stances than are these beautiful ivory cylinders and 

 plates, the crushing power of which, when worked by 

 the strong jaws, must be enormous, and sufficient to 

 prind the strongest shell that can be introduced between 

 them to powder. Although in all cases pavement- 

 like, the millstone differs considerably in the difl'ercnt 

 species in its structure. As an illustration of the group 

 we may take one of the millstones of the beaked eagle- 

 rays (Rliiuoptera), shown in Fig. 1. Here the millstone 

 is in the form of a semi-cylinder, consisting of five or 

 more rows of teeth ; a very usual number being seven. 

 Generally (as in the figure) the teeth of the middle row 

 are the widest; those of the rows on either side being 

 considerably narrower, while the two or three marginal 

 rows on each side may be compared to the tessara; in 

 a mosaic pavement. A further development of the same 

 type is exemplified by the typical eagle-rays {MyUohntis), 

 ic which the middle row of teeth in the millstone 

 becomes still wider, while the three lateral rows on 

 each side are reduced to the condition of hexagonal 

 tessarse. Moreover, whereas in the species of Rhiiu)batis 

 both millstones ai-e in the form of half-cylinders, in 

 Myliohatis the upper one alone retains this form, the 

 lower being a flattened plate. The culmination of this 

 type of structure is displayed in the rays belonging to 

 the allied genus Ai-fohnfin, in which both upper and 

 lower millstones are flat and composed only of the middle 

 row of teeth, which are of great width; the lateral rows 

 having completely disappeared. The existing represen- 

 tative of this genus is not very large (for a ray), seldom 

 if ever measuring more than about five feet across ; but 

 some of its extinct predecessors must have been monstrous 

 fish, as the teeth measure five or six inches in diameter. 



Quite a different type of dental armature is presented 

 by the millstones of the beaked rays {R/iinohatida;), of 

 which a specimen from the upper jaw of a species be- 

 longing to the genus Elnjnchohatis is shown in Fig. 4. 

 Here the teeth take the form of closely packed diamond- 

 shaped knobs, arranged in an alternating manner, so 

 that although they present longitudinal rows, yet they 

 also show oblicjue series, so as to give rise to a quincunxial 

 pattern. Then, again, the entire millstone in each jaw 

 is thrown into a series of undulations, so that the upper 

 one, as in the figure, exhibits a large median boss, 

 flanked by a pair of smaller undulations, which are 

 received into corresponding depressions in the lower 

 millstone. It is difficult to conceive a machine better 

 adapted for crushing than is presented by the jaws of 

 the beaked rays. 



Of a much" less powerful typo are the millstones of 

 the ordinary rays or skates (Raiidce) of our own coasts; 

 and among these the common thornback (Eaia clavata) 

 presents a very remarkable condition, since the indi- 

 vidual teeth take the form of obtuse knobs in the 

 female, whereas in the male the centre of each of these 

 knobs acquires a sharp recurved point. Since every 

 thing in nature has a meaning, it would seem a fair 

 inference that there must be some important difference 

 between the food of the male and female thornback, 

 but I have not come across any observations bearing 

 ujjo'i the sybject. 



