152 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 
- Fia. 16.— 
Fic. 14.— nodes. Mh arora Pseudoniscus. 
After Nieszkow- After Niesz- 
ski. Fic. 15.—Bnnodes. kowski. 
After F. Schmidt. 
Fe RSS 
ea) 
e 
Fic. 17,—Exapinurus. Af 
ter Nieszkowski. 
Fic. 18.—Hemiaspis. Af- 
ter Woodward. 
Fic. 19.—Neolimulus. After Woodward. 
After the foregoing classification was mostly written out, we found that Professor Zittel, in 
his excellent Handbuch der Palontologie, Bd. i, 640, 1885, has divided the suborder of Xiphosura 
into two families: 
1. Hemiaspide, with the following genera: Bunodes (Exapinurus Nieszk.) subgenus Hemiaspis, Pseudoniscus, 
Neolimulus, Belinurus, and Prestwichia; while Cyclus and ? Halycine are regarded as genera of uncertain position. 
2. Limulide, Limulus. 
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 a 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 Xiphosura, and aids in lending to Bunodes a trilobitic appearance. 
If we separate Bunodes from the true Xiphosura, Hemiaspis 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 asubgenus 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 Crustaceen fauna der Eurypteruschichten yon Rootzikiill auf Oesel. 
Mém. de Acad. impér. de St. Péterbourg, 7: ser., xxxi, 1883. 
Johnnes Nieszkowski, Zusiitze zur Monographie der Trilobiten der Ostserprovinzen nebst der Beschreibung 
einiger neuen obersilurischen Crustaceen. Dorpat, 1859. 
