Page:Proceedings of the Royal Society of London Vol 60.djvu/290

264 upper jaw, as in Anguilla. The m argin of the m onth is wide, as in Anguilla. The tongue is free, as in Anguilla. On the other hand, the youngest elvers which I have observed, have sm aller eyes than Leptoceplialus brevirostris, and this need not surprise us since we know th at in other species of Muraenoids the dim inution of the eyes occurs during the metamorphosis. . The nostrils are separated from one another, the anterior tubes are relatively at a considerable distance from the tip of the snout and from the rim of the mouth. They are in a position in which they are observed in many other Leptocephali, which are destined to transform themselves into adult forms having the anterior nostrils in nearly the same position as in the Common Eel. T he posterior nostrils, on th e contrary, are not tube-like, and are in the same position as those occupied in the adult Anguilla. It is worth rem arking th at in other Leptocephali also the posterior nostrils have already assumed the adult position when the anterior ones are still far removed from it. In L. brevirostris I find a larval dentition, which resembles th at of the other Leptocephali. In correspondence w ith the small size of Leptocephalus brevirostris the num ber of larval teeth is small. Researches founded, firstly, on the enum eration of the m yom eres; secondly, upon the enum eration of the dorsal and ventral arches of the vertebrae of the caudal extrem ity (hypurals) ; and, thirdly, upon the enum eration of the posterior spinal ganglia, lead with great certainty to the conclusion th a t the Z brevirostris is the larva of a Muraenoid, the num ber of whose vertebrae m ust lie between 112 and 117, most probably 114 or 115. Such a Muraenoid is the Anguilla vulgaris. The Muraenoid indicated cannot be any other of those occurring in the M editerranean, because they all have a num ber of vertebrae higher than 124* Counting the myomeres in Leptocephalus brevirostris one finds generally only 105 complete, five others incomplete, and all the others in a state of transparency and incomplete formation. These latter are fortunately a t the posterior extrem ity, where other criteria come to our assistance, namely, the spinal ganglia and the vertebral arches. To show how I arrive at the num ber of vertebrae which m ust be possessed by the adult individual, corresponding to a given Leptocephalus brevirostris, I quote the following example:—I assume th at three vertebrae develop themselves in correspondence to the first four incomplete myomeres, and th at 105 m ust develop themselves in relation to the 105 complete myomeres, th at is to say, between the fourth and fifth myomeres, between the fifth and sixth, and so on, until we reach the 105th vertebra, lying between the 104th and 105th myomeres. I

really occurs in the Mediterranean. The position of its nostrils and the number of its branchiostegal rays render its association with Leptocephalus brevirostris impossible.
 * Muroenesox savanna is said to have-109 vertebrae, but it is doubtful whether it