X 
INTRODirCTION. 
grounds. He conceives that “ the prototype of the Mollusca must 
have had a globose embryo attached to the apex, the apex composed 
of a living chamber opening into the protoconch or globose shell of 
the embryo, without septa, though possibly divided more or less by 
diaphragms. Diaphragms precede the formation of the septa in the 
embryo Ammonoid, and a cone formed by them appended to the 
bottom of the caecum was figured in the author’s ‘Embryology of 
Cephalopoda ’ (Bull. ATus. Comp. Zool., Cambridge, Mass., 1872) 
and described by Munier-Chalmas as the prosiphon. This confirms 
von Jheriug’s opinion that Tentaculites was the prototype of the 
Cephalopoda, since it has similar embryo and diaphragms.” 
Dr. S. P. Moodward, in the ‘ Manual of the Mollusca,’ inserts a 
footnote under the genus Orthoceras to the following effect : — that 
“ Theca and Tentaculites are provisionally placed with the Pteropoda, 
[but] they probably belong here.” Thus von Jhering was not alone 
in his opinion as to the affinities of Tentaculites^ though Professor 
James Hall still adheres to the view that its true relations are with 
the Pteropoda, adding, however, that these bodies “ in their com- 
paratively thick, calcareous test, and the annulating marks which 
affect the interior, and are visible upon the cast, are quite unlike 
the thin hyaline shells of most of the existing forms of Pteropoda ” h 
The development of the siphuncle is thus described by Professor 
Hyatt. Assuming that the ancestral forms of the Cephalopoda had 
closed cseca instead of a siphuncle, and that these were the “ initial 
stages” of the necks of the septa, he imagines that these caeca 
“ becoming prolonged in descendant forms, were differentiated into 
the funnels, the remnants of the caeca and the thinner walls of the 
sheath proper connecting them were formed by the fleshy siphon. 
This is the condition of the siphon in the typical forms and in 
the tubular-siphoned Orthoceras, but in some aberrant genera 
\_Endoceras and Piloceras~\ the fleshy siphon widens near the living 
chamber, becoming conical and forming a sheath. These sheaths 
lie in the large tube formed by the true funnels, and may deposit 
permanent diaphragms as in EndocerasT 
Since Hyatt wrote the foregoing new light has been thrown upon 
this subject by the discovery of the initial chamber of Endoceras, 
by Dr. Gerhard Holm, of Upsala^. The enormous size of this 
chamber, which is continuous with the siphuncular cavity, lends 
^ Pal. New York, 1879, vol v. pt. ii. p. 159. 
’ See p. 130 of this Catalogue. 
