304 NEW YORK STATE MUSEUM 



cumstances have permitted the retention of the smaller parts, 

 chiefly very young entire individuals or head shields of immature 

 forms, but have almost wholly destroyed larger bodies or left merely 

 patches and fragments of them. Thus this attempt encounters two 

 serious obstacles which may constitute two distinct sources of error : 

 (1) the effort to ascribe to very imperfectly known mature animals 

 a variant series of young stages; (2) the recognition of the char- 

 acter of mature adults and representatives of large genera from the 

 parts which have by accident escaped total destruction from shearing 

 and compression. Added to both of these difficulties is the consid- 

 eration that the appendages of all forms have rarely been observed. 

 A very determined effort has been made to overcome these defects 

 by the acquisition of copious material. The fossils are not abund- 

 ant. It has required the handling of a great many tons of rock 

 to acquire the half ton or so of specimens from which the selection 

 has been made for this descriptive account. The future will com- 

 plete our knowledge of the fauna : for the immediate present we may 

 content ourselves with the remarkable evidence it affords of the 

 age of the Shawangunk grit and of ontogenetic variations hitherto 

 unrecognized in this group of animals. 



MEROSTOMATA 



Order EURYPTERIDA 



Family EURYPTERID AE> 

 Genus EURYPTERUS 



The typical adult Eurypterus carries 12 tergites or dorsal seg- 

 ments and a telson. On the ventral side the number of segments 

 or sternites is reduced by one by the fusion of the first two into the 

 genital operculum. There have been few exceptions recorded to 

 this numerical value of the segmentation and it is generally recog- 

 nized as standing for the proper expression of complete segmenta- 

 tion in the entire family. It is interesting to note that the earliest 

 well known Eurypterid, Strabops thacheri Beecher, 

 described from a large entire specimen from the Cambric of Mis- 

 souri, carries but 11 segments. 1 This would be a phylogenetic 

 condition entirely compatible with the ontogenetic expressions pre- 

 sented by the material now before us. wherein we have very young 

 phases of Eurypterus with 11 and an extremely early stage of 

 Hughmilleria with apparently but 10 segments. 



1 Beeclier, C. E. Discovery of Eurypterid Remains in the Cambrian of Missouri. Ana. Jour. 

 Sci. 1901. 12:364. 



