314 ANNUAL OF SCIENTIFIC DISCOVERT. 



that, in fishes whose centre of gravity is far in front, the ventrals 

 seem to be carried forward the better to sustain them. If, with 

 large head and anterior ventrals, the pectorals are inserted far 

 back, the position of the latter is counterbalanced by their length. 

 In a few cases, jugular and thoracic fishes have an elongated body ; 

 but generally their odd fins are either very numerous or extended, 

 throwing the ventrals forward and making up in point of velocity 

 for their obtuseness of form. The dorsal may extend forward 

 even to the head ; the anal is never anterior to the ventrals ; the 

 anterior border of the dorsal apparatus is almost always in front 

 of that of the anal, sometimes on the same line, and rarely poste- 

 rior to it; it is very rare to find the whole dorsal behind the anal 

 {anaUeps). It is very common in jugular and thoracic fishes to 

 find the posterior border of the dorsal and anal on the same line; 

 while, in most of the abdominal fishes, the anal is wholly behind 

 the dorsal. 



Absence. — The ventrals are the most frequently deficient, as in 

 the eel family ; in other families, the absence of the ventrals is fully 

 compensated by the length and suppleness of the body or the great 

 development of other fins. The occurrence of ventrals with ab- 

 sence of the pectorals, on one or both sides, is very rare (pleu- 

 ronectes). The simultaneous absence of the pectorals and ventrals 

 occurs in ophichthys. The odd fins (dorsal and anal) are, there- 

 fore the last to disappear. — Gouriet, Comptes Reiidus, June, 1868. 



HOMOLOGIES OF THE TEETH OF THE MAMMALIA. 



Mr. Flower stated, in a paper before the British Association in 

 1868, that he had discovered that some of the so-called monophyo- 

 dont mammals, as the armadillos, had a complete set of milk 

 teeth preceding those with which they are provided when adult ; 

 and that in marsupials only one tooth, the so-called fourth premo- 

 lar, has a successor, while in dogs only one, or at most two, of 

 the premolars have predecessors. In seals the first set of teeth 

 are extremely minute, mere points, and this led to the belief that 

 they were evanescent. He concludes that the so-called milk, or 

 first set of teeth is by no means to be regarded as the typical 

 series to which the second set are added, as Owen maintains; but, 

 on the contrary, that the milk teeth are something added to the 

 normal dentition in the higher mammals, and more especially in 

 those groups which require teeth when young. It is a question 

 whether the evanescent condition of the milk-series of teeth in the 

 seals indicates an approach to the condition of the cetacea, which 

 have but one set, or are monophyodont. 



SIREDON, A LARVAL FORM OF AMBLTSTOMA. 



Dumeril in 1867, from changes observed at the '* Garden of 

 Plants," in Paris, maintained that the Mexican axolotl is a larval 



