26 William Patten. 



that the large anterior incision served for the attachment of an oar-like appendage similar 

 to that in Pterichtliys and Bothriolepis. The remaining openings, which are unquestionably 

 sei'ially homologous with the first, must have served for the attachment of other appendages 

 of a similar nature. They decreased in size from before backwards, and were possibly too 

 delicate to be 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- 

 lar 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 interpretation 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 Ojatfiaspis 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 

 region 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 have 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- 

 derms can be best explained, aswe have 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 Tremataspldae and the Arthropods: The following 

 summary of the principal characteristics of the Tremataspldae shows how strongly they re- 

 semble the Arthropods, and how surprisingly the resemblance has been strengthened at cer- 

 tain points by important discoveries in unexpected directions. 



