APRIL 11, 1884.] 
may appear, is by no means satisfactory. The 
writer of the letter, in which he may have en- 
closed a stamp, though this is supposing an 
extreme case, receiving no answer, feels himself 
aggrieved, and writes again ; so that in the end 
the receiver is forced to answer to protect him- 
self. Isthere, then, noremedy? Perhaps not. 
We nevertheless appeal to the public to bear 
in mind that the college-professor, however 
little he may have to do (and it is well known 
that this is very little), has at least something 
to do besides answering every question regard- 
ing business-matters in which it is thought that 
his advice may be of aid. Ask him any thing 
you please in the interests of matters pertain- 
ing to education or pure science, but draw the 
line when it comes to asking for what may 
fairly be called ‘ professional advice,’ in the 
sense in which that expression is used by the 
lawyer and the doctor. 
Two of the most unexpected discoveries in 
the deep-sea soundings during the last cam- 
paign of the Talisman, under the supervision 
of Prof. A. Milne-Edwards, are, first, the 
discovery of polished and scratched pebbles 
at a depth of five thousand metres, between 
the Azores Islands and the coast of France, 
indicating plainly the existence there of ice- 
bergs during the glacial epoch; and, second, 
of stones with impressions of parts of trilo- 
bites also brought up by the trawls. If these 
rocks with trilobites belonged where found, it 
will go far to prove the existence of an Atlantis 
continent during the secondary and tertiary 
epochs. 
As a rule, one would not expect scientific 
knowledge to be much advanced, or very use- 
fully diffused, by elegant extracts and quota- 
tions. But in a small book just issued by 
Appleton & Co., made up of ‘ characteristic 
passages from the writings of Charles Darwin,’ 
Mr. Nathan Sheppard has really produced, in a 
form at once authentic, brief, and inexpensive, 
an instructive and very readable account of 
Darwinian doctrine in the words of its found- 
er. The pieces are put together with no small 
SCIENCE. 
429 
skill, not in the order of publication, but 
rather in the order of evolution. It begins 
with the movements and habits of plants, rises 
from these to worms, discourses of the varia- 
tion and struggle for existence of the higher 
living forms, and so to the highest, — 
‘The diapason closing full in man.’ 
LETTERS TO THE EDITOR. 
*.* Correspondents are requested to be as brief as possible. 
The writer’s name is in all cases required as proof of good fuith. 
The relations of Didymodus, or Diplodus. 
My reverence for the genius of Professor Cope is 
so great, and my confidence in his acumen so implicit, 
that when he assured me, first personally, and then 
in Science (iii. 275), that Didymodus (a substitute 
for Diplodus) was the proper name for Chlamydo- 
selachus, I was willing to at least concede that the 
two forms might possibly be related. Knowing, as I 
did, that different types had been confounded under 
the name Diplodus, I was content to await the pub- 
lication of Professor Cope’s views before expressing 
a positive opinion, thinking he might have evidence 
in reserve which would gainsay what had been before 
offered. A résumé of Professor Cope’s observations 
has just appeared, as promised, in the American natu- 
ralist for April (xviii. 412, 413), and we are therefore 
in a position to test his utterances. Notwithstanding 
the reverence and confidence that I have expressed, 
I can but think now, that for once Professor Cope 
has been too hasty, and tripped. Iam convinced, not 
only that Didymodus has no generic nor even family 
relations with Chlamydoselachus, but that it repre- 
sents even a different order. My belief in Professor 
Cope’s candor equals my other sentiments, and I pre- 
sume he will discard his first-formed opinion when 
his attention is called to certain facts. 
The history of Didymodus, or Diplodus, is a long 
one, and is complicated with that of several others. 
I need only give the salient features. 
In 1887 Professor Agassiz (Poiss. /oss., ili. 66) de- 
scribed a spine which he believed to have belonged 
to a fish like the sting-rays, as Pleuracanthus laevis- 
simus. The only example was obtained from the 
Dudley coal-field. 
In 1845 Professor Agassiz (Poiss. foss., iii. 204) 
made known certain teeth, which he referred to 
sharks of the family of Hybodonts. ‘Two ‘ species’ 
were distinguished, D. gibbosus and D. minutus. 
Both were obtained from the English coal-measures. 
In 1848 Professor Beyrich (Berichte verhandl. k. 
preuss. akad. wiss., 1848) proposed the generic name 
Xenacanthus for a German carboniferous form re- 
ferred to Orthacanthus by Goldfuss (1847), but which 
approached nearer to Pleuracanthus. 
In 1849 Dr. Jordan (Jahrbuch fiir min. u. geol., 
p. 843) described, under the name Triodus sessilis, a 
form subsequently ascertained to be identical with 
the Xenacanthus. 
In 1857 Sir Philip de Malpas Grey Egerton (Ann. 
and mag. nat. hist., xx. 423) contended that the 
spines of Pleuracanthus belonged to the same fish 
as the Diplodus teeth, and that Xenacanthus was 
likewise referable to the same type. 
In 1867 Professor Kner (Sitzb. k. akad. wiss., 
ly. 540-584) published an elaborate memoir, illustrat- 
ed by ten plates, in which he proved conclusively 
