26 
William Patten. 
that the large anterior incision served for tke attacliment of an oar-like appendage similar 
to that in Pterichthys and Bothriolepis. The remaining openings, which are unquestionably 
serially komologous with the first, must hâve served for the attachaient of other appendages 
of a similar nature. Tliey decreased in size from before hackwards, and were possibly too 
délicate to he well preserved in a fossil condition. 
J. The Entapophyses, and the Trabeculae. It is doubtful whether the so called endolym- 
phatic ducts are actual perforations of the shell. In sections they are seen to be deep, tubu¬ 
lär infoldings, probably closed at the inner end. They are thus comparable with those in- 
foldings, or entapophyses, on the dorsal shield of Limulus, which serve for the attachment 
of dorso-ventral muscles. This interprétation is strengthened by the fact that serial sections 
show the presence of two more bony ingrowths, just behind and in line with the so called 
endolymphatic ducts. The second pair of entapophyses are long deep plates that unquestion¬ 
ably serve for the attachment of muscles, since the frayed out ossified tendons are still at- 
tached to their inner ends. These entapophyses are of great importance as they indicate a 
similar arrangement of muscles and appendages to that seen in Limulus. They probably 
served for the attachment of dorso-ventral muscles, arising from the dorsal surface of the 
cartilaginous cranium and from the various pairs of cephalic appendages. 
The presence of a similar set of muscles in Pteraspis and Cyathaspis is indicated by 
the radiating impressions, or lobes, on the inner surface of the shell, similar to the radiat- 
ing muscle markings on the dorsal wall of Limulus. 
Transverse sections of the whole head show that the shell is specially thickened in the 
région of the crista occipitalis, and that the margin of the shield is considerably strength¬ 
ened by a network of bony trabeculae uniting the edges of the dorsal and ventral walls. 
These marginal trabeculae are very similar to those I hâve described in Limulus. The 
marginal cells of Eukeraspis are probably produced by a special arrangement of these 
trabeculae. 
A loose network of bony trabeculae is also developed round the median and lateral 
openings of the dorsal shield, and along the incisions of the ventral wall, where the append¬ 
ages are attached. With the above exceptions, the inner surfaces of the dorsal and ventral 
shields are smooth. 
The minute structure of the shell of Tremataspis and other members of the Ostraco- 
dernis can be best explained, aswe bave pointed out elsewhere, on the assumption that it 
is a modification of a three layered dermal skeleton similar to that of Limulus. 
K. The Resemblance between the Tremataspidae and the Arthropods: The following 
summary of the principal ckaracteristics of the Tremataspidae shows how strongly they re¬ 
semble the Arthropods, and how surprisingly the resemblance lias been strengthened at cer¬ 
tain points by important discoveries in unexpected directions. 
