346 THE OSTRACODERMS AND THE MARINE ARACHNIDS. 



thorax of certain trilobites, and contended that the ostracoderms were the connect- 

 ing links between the arachnids and true vertebrates; and in 1894 he pointed out 

 the extraordinary resemblance in the microscopic structure of the shield of Pteras- 

 pis and other ostracoderms and that of Limulus. These resemblances in the min- 

 ute structure of the shields were either ignored by later writers, or regarded as 

 mere coincidences, or as the results of mimicry or of " parallelism." 



Lankester, Woodward, Traquair, Rohon and others, agree in denying the 

 existence of arthropod characters to the pteraspids, apparently because of the 

 abundant evidence now available that Pteraspis is related to Cephalaspis, whose 

 ichthyic affinities have rarely been questioned, rather than because the arthropod 

 features of Pteraspis have been dispassionately considered and found wanting. 



But within recent years there seems to be a growing tendency to doubt the 

 affinity between Pteraspis and Cephalaspis. Reis protests against their union, 

 and apparently Traquair is in doubt, treating them together largely as a matter of 

 convenience. Lankester in his earlier monograph states that "The heterostraci 

 are associated at present with the osteostraci because they are found in the same 

 beds, because they have, like Cephalaspis a large head shield, and because there is 

 nothing else with which to associate them." More recently he has said (1897) 

 "There is absolutely no reason for regarding Cephalaspis as allied to Pteraspis 

 beyond that the two genera occur in the same rocks, and still less for concluding 

 that either has any connection with Pterichthys." Zittel says, Vol. Ill, page 147, 

 "Mir scheinen die Beziehungen der Pteraspiden und Cephalaspiden nach Form 

 und Structur so entfernt dass beide besser als besondere Ordnungen betrachtet 

 werden." He remarks further on that while the cephalaspidae certainly appear to 

 be ganoids, the position of the pteraspidae is very doubtful. 



Muscle Markings. In 1872, A. Kunth described in Cyathaspis integer a 

 series of six "flache Hocher," situated on the under surface of the neural shield, 

 which he regarded as indications of segmentation. Lankester (1873), describes 

 similar impressions on the shield of Cyathapis banksii and believes that in both 

 cases they indicate the position of a series of branchial chambers. In Pteraspis 

 also, Lankester has described five narrow ridges, with four broad shallow 

 depressions between them, which radiate from the center of the inner surface 

 of the neural shield. They are perhaps best marked in Pteraspis crouchii and 

 P. rostratus. These markings I have explained as indications of the original 

 segmentation of the mesocephalon, produced in part by the attachment of strong 

 segmental muscles extending vertically from the inner surface of the neural shield, 

 either to a cartilaginous cranium, or to a series of gill-like, or jaw-like, segmental 

 appendages on the haemal side. (Fig. 244, M.) 



The following quotation illustrates the attitude of modern paleontologists 

 toward the ostracoderms. A. S. Woodward, in his text-book of Paleontology 

 (1898, page 5) states that "Nearly all the genera mimic in a curious manner 

 the contemporaneous eurypterids;" and on page 24 of the Introduction, that 

 "The oldest ostracoderms, sometimes claimed as the immediate allies of the 



