702 The American Naturalist. [August, 
preserved in the Museums of St. Petersburg, London, and Wash- 
ington. 
The lines of descent of the Sirenia may be expressed as fol- 
lows: 
Manatus. Miosiren. Rhytina. 
Halitherium. Halicore. 
Dioplotherium. 
* 
Prorastomus. . 
x 
EXPLANATION OF PLATES. 
PLATE XXV.—Dioplotherium manigaultii Cope. Three-eighths natural 
size. Figs. 1-4, part of premaxillary bone with incisor alveoli, and I. 1 in 
place; 1 outside; 2 inside; 3 front; 4 from below; a alveolus of external 
incisor. Original; from type in Museum of Charleston, S. C. Figs. 6-7, 
os innominatum of unknown Sirenian, from Charleston. Reduced. 
PLATE XXVI.— Ha/icore dugong Cuv. Skeleton, from Cuvier. 
THE CONCRESCENCE THEORY OF THE VERTE 
BRATE EMBRYO. 
BY CHARLES-SEDGWICK MINOT. 
(Continued from page 629.) 
Blastodermic Vesicle with Two Layers—Of this stage we have 
several descriptions; for the rabbit by Kölliker (Grundriss, p. 89), 
Hensen, 24, C. Rabl, 44, 141, as well as the older accounts by 
Bischoff, 6, and Coste, rz, and the brief mention by Heape in 
Foster and Balfour's Embryology, 2d edition, 316-320; for the 
mole by Heape, 23; for the dog by Bischoff, 7; for the cat by 
Schafer, 52; for the sheep by Bonnet, zo; and for several ro- 
dents as indicated in the section on inversion of the germ layers, 
Ky 
