1896. ] NEW YORK ACADEMY OF SCIENCES. 163 
They certainly do not correspond to the basalia of the dorsal 
fin shown in the present material, although they may have rep- 
resented the external rays. It is possible that they may prove 
to have been caudal structures. 
The additions to a knowledge of the structures of Dinichthys 
as given in the present paper, may be summarized as follows: 
I. Presence of notochordal axial skeleton, of dorsal fin and 
of pelvic girdle, similar to those in Coccosteus. 
II. Presence of an hitherto undescribed pair of dermal plates. 
III. Evidence that a pair (and probably, therefore, all) of the 
plates of the ventral armoring were surface plates. 
A contrast of the ventral armoring of Dinichthys gouldi with 
the corresponding structures in D. terrelli and in Coccosteus (cf. 
Pl. VIII., Figs. 1,2 and 3), leads the present writer to believe that 
the conditions in Coccosteus are most nearly those of the com- 
mon ancestor of these three forms. And that in D. gouldi are 
present more generalized conditions than in the larger species. 
Thus in Coccosteus the plastron is composed of separate dermal 
plates, more or less rhomboidal in outline, and differing little 
from each other in size. In Dinichthys, on the other hand, these 
plates differ more widely from each other in size and outline, 
the median plates—median ventral and anterior median ventral 
—no longer broadly lozenge-shaped and separate, are now re- 
duced toa single narrow lanceolate element. In D. gouldi the 
plates of the plastron overlap as in Coccosteus; in D. terrelli 
they become more widely unlike each other and are fixed to- 
gether with definite sutures. 
May 12, 1896. 
THE HISTORY OF THE ACHROMATIC STRUCTURES 
IN THE MATURATION AND FERTILIZATION 
OF THALASSEMA. 
By Brapney B. Grirrin, B. Sc. 
University Fellow in Biology, Columbia University. 
The following observations on the eggs of the chetiferous 
gephyrean, Thalassema, were made on a series of stages col- 
lected at Beaufort, N. C., by Professor E. B. Wilson, in the 
summer of 1895, with a view to the investigation of the achro- 
matic structures, and especially the centrosome in maturation 
and fertilization. In intrusting this material to me Professor 
Wilson suggested that I should first follow out in detail the 
history of the centrosome with special regard to its behavior 
