152 



MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



Fig. 14. — nodes. 

 .After Nieszkow- 

 ski. 



Fig. 15.— Bunodes. 

 Aftei F. Schmidt 



Fig. 16.— 

 Psendouiscu.'*. 

 After Niesz- 

 kowski. 



r \ 



Fig. 17. — Exapinnrus. 



ter Nieszliowski. 



Fig. 18.— Hemiaspis. Af- 

 t«r Woodward. 



Fig. 19. — Neolimulus. After Woodward. 



After the foregoing classification was mostly written out, we found that Professor Zittel, in 

 his excellent Handbuch der Palseontologie, Bd. i, 640, 1885, has divided the suborder of Xiphosura 

 into two families : 



1. Uemiasplda; with tlie foUewing genera: Bnnodes (Exapiuuius Mieszk.) subgenus Hemiaspis, PseudouiaciLs, 

 Neolimulus, Belinurus, and Piestwichia; wliile Cyclus and f Halycine are regarded as genera of uncertain position, 

 y. Limulidai, Liuiulns. 



It seems to us that this is scarcely a natural classification, and that it would be better to sep- 

 arate the Silurian forms mentioned above from the genuine Xiphosura, especially as we know noth- 

 ing of the nature of their appendages, and to assign them, at least provisionally, to a group dis- 

 tinct from the genuine Xiphosura, especially since we now know something definite as to the 

 nature of the cephalic appendages of Cyclus and Prestwichia, their resemblance to those of the 

 existing Limuli being remarkably close. Certainly Bunodes, in which there are, according to F. 

 Schmidt's late researches,* as stated and figured by Zittel, besides a four-jointed abdomen, a "tho- 

 rax" composed of "six trilobite-like, movable segments," cannot well be allowed ti position in the 

 genuine Xiphosura. Moreover, the pleura of the single segments show a diagonal longitudinal 

 ridge. This mark is a peculiarity of the pleura of some trilobites, and does not occur in any genu- 

 ine Xipho.sura, and aids in lending to Bunodes a trilobitic appearance. 



If we separate Bunodes from the true Xiphosura, Hemiasi>is will have to go with it, since it 

 has a rounded cephalic shield, shaped somewhat as in Bunodes, but broader. We should not, with 

 Zittel, regard it as a subgenus of Bunodes, because the " thoracic" segments have on the free sides 

 no diagonal ridge, and the cephalic shield is ornamented with large spines, which perhaps indicate 

 the head segments of the embryo. In both genera no eyes have yet been discovered. For the 

 present we should, on the whole, regard the two genera as representing different families. 



*F. Schmidt, Miscellanea Silurica III. Die Crustaceea fauna der Eurypteruschichten von Rootzikiill auf Oesel. 

 M6m. de I'Acad. inip^r. de St. P^terbourg, 7" ser., xxxi, 1883. 



Johuues Nieszkowski, Zusiitze zur Monographie der Trilobiten dor Oatserprovinzen uebst der Besebreibuug 

 einiger neuen obersilurischen Crustaceeu. Dorpat, 1859. 



