86 Journal Xew York Entomological Society, [^'o'- >^>^i>^' 



limbs) of a trilobite such as that shown in Fig. 9 is more Hke that 

 of the mouthpart limits of the merostomes (Figs. 12, 8, etc.) than it 

 is like the mandible of a crustacean, as was pointed out above, and 

 in addition to the similarity in the head region, and in the body seg- 

 mentation and the pygidial region of an immature merostome such as 

 Lhnulus and certain of the Trilobita. this resemblance might justify 

 our grouping the trilobites with the merostomes, etc., in the sub- 

 phylum " Podognathata," rather than with the Crustacea and their 

 allies, in the subphylum '" Eugnathata.'' The gnathobase "" gh " of a 

 trilobite such as that shown in Fig. 18 is very similar to the gnatho- 

 base " gh" of the merostome shown in Fig. 12, and the appendage 

 homologous with the mandibular appendage of Crustacea, etc., is 

 more like a walking leg in the trilobites. merostomes, and their allies, 

 than it is like a true mandible (such as the mandible of a crustacean), 

 so that the evidence of the mandibular appendage of trilobites and 

 merostomes would indicate a somewhat closer relationship between 

 these two groups than between the trilobites and Crustacea, although 

 the trilobites are clearly intermediate between the Crustacea and 

 Merostomata, and are but slightly nearer the one than the other. 



In the eurypterid shown in Fig. 17, there is a small appendage 

 " ci" called the '■ epicoxite " 'by Clarke and Ruedemann, 1912, which 

 is situated immediately below the gnathobase " gh " of the fourth 

 '■ endognathite "' (or mesal region of the basal segment of the limb) 

 according to Clarke and Ruedemann. In the limb of the xiphosuran 

 Limnlus shown in Fig. 14, I find a similar structure " ci" situated 

 below the gnathobase " gh," and if the structure " ci" of Fig. 14 is 

 homologous with that labeled " ci" in Fig. 17 (as seems to be the 

 case) the presence of this peculiar structure in both eurypterids and 

 xiphosurans would further strengthen the view that the two groups 

 are extremely closely related. 



Since the Scorpionida are apparently descended from forms re- 

 sembling the Eurypterida and other merostomes. I have tried to find 

 the homologues of the gnathobases " gh" of Figs. 15 and 12, in a 

 scorpion's limb and I would suggest that the small projection labeled 

 " gh" in the basal segment of the limb of the scorpion shown in Fig. 

 16 may correspond to the gnathobases " gh" of Figs. 15 and 12. 



