346 The American Naturalist. [April, 
tal retinular region. The principal differences between these eyes and 
the middle eyes of the scorpion lie in the absence of a central cell, in 
the anatomy of the retinule and in the absence of inter-retinular pig- 
ment cells from the Phalangids. As a summary Purcell says: “The 
anterior middle eyes of the spiders, the eyes of Phalangids and the 
middle eyes of the scorpions, as well as the middle eyes of Limulus, 
represent a series of homologous structures, which are characterized 
by an inverted retina with retinule or at least rhabdomes.” 
Range of Placostylus.—A study of the geographical distribu- 
tion of the land molluscan Placostylus, by Mr. C. Hedley, leads to 
some interesting conclusions. According to that author, Wallace's 
theory of a land connection between Australia and New Zealand is 
untenable. Mr. Hedley’s theory is that the various islands where 
Placostylus is found, embracing the archipelagoes of Solomon, Fiji, 
New Hebrides, Loyalty, New Caledonia, Lord Howe and New Zealand, 
are the remnant of a continental area to which he gives the name, 
Melanesian plateau. This plateau was never connected with nor popu- 
lated from Australia ; its fauna was probably derived from Papua vi% 
New Britian. New Zealand and New Caledonia were early separated 
from the northern archipelagoes, while the Fijis remained to a later date 
in communication with the Solomons, but were severed from that group 
before the latter had acquired from Papua much of its present fauna. 
The author calls attention to the fact that not the depth but the per- 
manence of the ocean is the real limit to the distribution of the forms 
of life. (Proceeds. Linn. Soc. N. S. W., 1892). 
The Scales of Lepidosteus.‘—Mr. W. S. Nickerson finds that 
in Lepidosteus the dermal scleroblasts give rise to three different pro- 
ducts: (1) calcareous scale material, (2) ganoine, and (3) a ganome 
membrane. There is no differentiation of the cells, but rather 4 modi- 
fication of the function of the same cells at different periods of thelr 
history. The ganoine has been called the enamel layer,, but 1t 18 r 
enamel, as its development and chemical reactions show. Its secret x 
on the outer surface of the scale by cells of dermal origin, not by ep! 
During the development of the scale, spines tipped with an a 
layer are formed, but disappear before the maturity of he 
Their number and irregularity of distribution over the scale oppose? 
*Bull. Mus. Comp. Zool., XXIV, No. 5 (1893). 
