86 NEW YORK STATE MUSEUM 



Hemiaspidae, as proposed by Scliinidt. We briefly notice the 

 distinctive characters of these two genera. 



In P s e u d o n i s c u s the head-ehield is relatively very larger- 

 covering one half the length of the animal. It has also sharp- 

 and extended genal spines. In B u n o d e s the head plate is 

 short, sub-semicircular and blunt at the angles. In P s e u d o - 

 n i s c u s the surface of the head is generally smooth, but in one 

 specimen from the dolomites of Litchfield it shows traces of 

 radial grooves departing from the median region. These seem to« 

 us probable impressions of appendages. In B u n o d e s, how- 

 ever, such Tadial depressions are very pronounced in the speciee 

 B. lunula and r u g o s u s, but are less prominent in B. 

 lunula var. schrencki^ (Exapinurus schrencki 

 Nieszk.) I see no objection to placing a construction similar to 

 the above on these cephalic grooves in B u n o d e s. Schmidt 

 says that clear evidences of facial sutures are present in the- 

 Hemiaspidae, and cites in confirmation one of his figures of 

 B u n d e s (pi. 1, fig. 34). These are said to converge in direct 

 lines from the posterior to the anterior margin, but the cited 

 figure seems to throw no certain light on this structure, and we 

 have seen in all the specimens of Pseudoniscus studied no 

 trace of such suture on the surface of the head. Our specimens, 

 however, show an infolded margin or doublure thickened at its 

 inner edge by a thread-like line. This line parallels the margin 

 of the shield except at the median projacent anterior angle on 

 the shield^ where its lateral branches join the margin at the apex 

 of the angle. This may prove to be a submarginal line of suture. 



In the structure of the abdomen the most striking feature in. 

 both genera is the very strong trilobation and the termination 

 in a sharp telson. Dr Schmidt has (perhaps through a lapsus 

 calarm) referred to this aspect of these creatures as a ^^ trilobi- 

 tation", a most expressive term for the strong resemblance herein 

 presented to the trilobites. The dorsal furrows are deeply 

 marked, the axis arched and the pleurae flat T\ith pointed termi- 



^See Schmidt Die Crustacean-fauna der Eurypterus-schichten von 

 Bootzilull auf Oesel, pi. 1, fig. 34-47, pi. 7, fig. 1-6. 



