Miscellaneous. 419 
Thus we have undoubted evidence of the existence of Pa- 
chastrella in the Cretaceous period, but none such of a Calci- 
sponge. 
MISCELLANEOUS. 
On the Young Stages of some Osscous Fishes: Development of the Tail. 
By Mr. Au. Agcasstz. 
In this valuable memoir the author refers to observations made 
in great detail principally upon Pleuronectes. The conclusions at 
which he arrives are of great interest in connexion with the succes- 
sion of forms among fossil fishes. 
The young Plewrvnectes, on escaping from the egg, has the pos- 
terior extremity of its dorsal chord straight, and its caudal fin is 
rounded. Soon the extremity of the dorsal chord becomes greatly 
arched upwards, presenting a concavity towards the ventral surface; 
at this moment appears the first trace of separation between the em- 
bryonic caudal fin and the permanent caudal, as also the first indica- 
tions of the principal and accessory rays of the tail. In the succeeding 
period the emargination which exists between the embryonic and 
the permanent caudal has become deeper and the chord is more 
arched, the rays are well marked, and the permanent caudal extends 
backward beyond the embryonic caudal. 
In the further course of development the embryonic caudal 
acquires the form of a large independent lobe; the permanent fin 
extends entirely beneath it, and forms a distinct fin having the ap- 
pearance of a second anal. On arriving at this point the caudal 
region of a young Pleuronectes presents a striking resemblance to 
that of the young Lepidosteus as described and figured by Wilder *. 
The arch formed by the caudal extremity of the chord becomes 
still more strongly marked ; and the permanent caudal extends at 
first as far as the embryonic caudal, and afterwards passes beyond 
it. The embryonic caudal is thrown more and more upwards ; and 
the rays of the permanent caudal acquire a fan-like arrangement. 
While these latter changes are taking place we observe a disap- 
pearance of the notochord, which precedes the formation of the 
urostyle. The embryonic caudal also disappears more and more, 
and finally only presents the form of a slight semitransparent thick- 
ening of the dorsal line. On the other hand, the permanent caudal 
increases continually ; from being pointed it becomes rounded, and 
acquires the form seen in the adult. At the same time the ossifica- 
tion of the vertebree takes place, the cartilages which support the 
rays appear, &c. 
Mr. A. Agassiz has observed these same phases of development of 
the caudal fin in a great number of other genera of fishes belonging 
* “Notes on the American Ganoids,” Proc. Amer. Assoc, Ady. Sci. 
1876 (Detroit Meeting). 
