ANATOMY AND RELATIONS OF THE EURYPTERIDS. 523 



Relation to Tkilobita. 



In our present state of comparative ignorance as to the details of the different 

 appendages of Trilobites, any attempt at comparing them with Eurypterids must be more 

 or less superficial. The form of the body presents certain points of resemblance, inasmuch 

 as it consists of a carapace followed by a number of free segmeDts, and ending in a telson. 

 The carapace probably corresponds to that of Eurypterus, &c, and shows in some forms 

 indications in the glabellar furrows of five segments,* or, if one counts the frontal lobe, 

 of six. The lateral eyes are situated on the dorsal surface ; and unless we consider the 

 margin of the carapace to be the facial suture, this is, as mentioned above, an argument 

 for considering Eurypterus as a more primitive form than Pterygotus. If the facial 

 suture be taken as representing the margin of the carapace, then the free cheeks probably 

 correspond with the inturned portion. The presence of central eyes must be held as not 

 yet proven, though I think Woodward's t interpretation of the small openings in the 

 glabella as central eyes is probably correct. 



The number of free segments in the lower Trilobites is usually greater than in 

 Eurypterids, but one sees within the group itself how easily the number of segments in 

 such comparatively unspecialised forms can be increased or diminished. The structure of 

 the segments is more important, and here there seems to be very little resemblance 

 between Trilobites and Eurypterids, as the latter show no trace of pleurae, unless indeed 

 the epimera of some species of Stylonurus may be regarded as much reduced pleurae. 



What is known of the appendages affords little ground for comparison. The maxilla 

 and palp, described by Woodward in Asaphus platycephalus,\ are not unlike the first 

 postoral appendage of Eurypterids. If they correspond to this last, there ought to be a 

 preoral pair which are probably concealed beneath the hypostoma, which would corre- 

 spond to the epistoma of Eurypterids. The traces of appendages in Asaphus platy- 

 cephalus § and Asaphus megistos || show little of importance for comparison. Far more 

 suggestive is Walcott's restoration of Calymene senaria H with the last larger pair of 

 thoracic appendages. His restoration of a transverse section of a body segment, however, 

 shows nothing comparable to what is found in Eurypterids. 



It must be remembered, that what little is known as to the anatomy of the Trilobites 

 is almost entirely based on the more highly specialised forms. If we could get reliable 

 information as to the appendages of such forms as Olenellus or Paradoxides, there would 

 be some fair chance of comparing them successfully with Eurypterids. 



Relation to Crustacea. 



That the Eurypterids are usually classed with Crustacea, must be ascribed to their 

 aquatic habit and branchial respiration. It is difficult to free one's mind of the idea that 



* Olenellus Callavei, Lapworth, Geol. Mag., III. viii., pi. xv. + Geol. Mag., 1883. 



| Quart. Jour. Geol. Soc, vol. xxvi., 1870. § Billings, Paleozoic Fossils of Canada. 



|| Walcott, Bui. Mus. Comp. Zool, Harvard, 1881. 1 Ibid. 



