Z PROCEEDINGS OF THE AMERICAN ACADEMY 



part to the anal opening. Their general resemblance, at this time, to 

 the Ganoid types of older periods, and especially to the Amias of the 

 present day, cannot be too strongly insisted upon. In the Flounders, 

 there is usually but a single dorsal and anal fin, formed from the 

 original embryonic fin-fold. I will only notice, in a general way. the 

 separation of the anal and dorsal from the caudal, by the earlier ap- 

 pearance of the permanent fin-rays ; and the more rapid growth of the 

 caudal, during the time when, in the dorsal and anal fin, the em- 

 bryonic fin-rays, which disappear with the growth of the permanent 

 ones, are still the most prominent feature. Little by little, however, 

 with the increase in depth of dorsal and anal (PL IV. figs. 2, 3, 4, 5), 

 the separation between these and the base of the caudal becomes more 

 abrupt ; and this, accompanied by the gradual shrinking of the rem- 

 nant of the embryonic fin-fold at the base of the caudal both above 

 and below, soon brings the relations of the three principal fins of the 

 Flounders to the proportions they bear in the adult (PI. IV. fig. 5). In 

 another species (PI. IX.), I shall describe the gradual development of 

 the anterior dorsal out of the primitive embryonic fin-fold. In the 

 bony fishes, neither the development of the ventrals nor the pectorals 

 has yet been traced from a lateral embryonic fin-fold ; but, in sharks 

 and skates, the case is different. (See J. Wyman,* in his development 

 of Raja.) 



We may perhaps find hereafter, in the development of such forms as 

 Lumpus, Liparis, and the like, a nearer approach to the Selachian mode 

 of development of the paired fin-rays. In those of the bony fishes 

 the development of which I have had an opportunity of following, the 

 pectorals are well developed ; early assuming, even while in the egg, 

 the Ganoid (Crossopterygian) type, to which I have already alluded in 

 the first part of this paper.f In some of the earlier stages, the lat- 

 eral embryonic fold, from which the pectorals are formed, can be dis- 

 tinctly traced, — though never assuming the great prominence which 

 it has in the dorsal or anal embryonic folds, the paired fins early con- 

 cealing the lateral folds ; while it is the reverse with the dorsal and 

 anal folds, from which the dorsal and anal fins are developed late. 



The ventrals, on the contrary (PI. VI. fig. 5, PL VII. fig. 4, PL IX. 

 fig. 6), make their appearance very much later, and, in our Flounders, at 



* Wyman, Jeffries. Observations on the Development of Raja Batis, 

 in Mem. Am. Acad. Boston, 1864. And also Balfour, F.M. Elasmobranch 

 Fishes. 



t Agassiz, Alexander. On the Young Stages of Osseous Fishes. Proc, 

 Am. Acad. xiii. Boston, 1877. 



