GRAPTO.LITES OF NEW YORK, PART 2 345 



suture marks one of the faces ol the rhabdosome throughout the 

 greater part ol its length, except in the neighborhood of the sicular 

 (proximal) extremity, where for some distance the test is continuous and 

 unbroken. 



Each theca is of the form oi a subprismatic tube about three or four 

 times as lone;' as wide, and with a longer ( back to front) diameter about 

 twice that of the; shorter (lateral) width. The lower part of the theca is 

 crushed inwards forming a well marked "excavation." The upper part is 

 free and somewhat sacculate. It is limited above by a horizontal apertural 

 margin. This margin is frequently edged by a thickened rim or " welt " 

 and is furnished at each of its corners with a stumpy horizontal spine. 



The surface of the rhabdosome is smooth throughout. 



This especial type of rhabdosome naturally admits of becoming com- 

 pressed in a variety of different aspects which at first sight appear to be 

 characteristic of very distinct species. 



{"■) In the ordinary or m\d-/>roJi/c view (the sutural line being exactly 

 central in position on the specimen) the rhabdosome becomes compressed 

 symmetrically, the margins of the rhabdosome are constituted by the mid sec- 

 tion of the theca. This is Murchison's original view [see Silurian System ]. 

 The spines are invisible but the compressed "welt" projects in an upward 

 and inclined direction as a well marked "mucro." 



The apertural margin is concave and the lateral margin convex above 

 and bent into the excavation below. 



(,5) In the quarterface view, a very frequent one, we see the rhabdo- 

 some so compressed that the left spinose corner of the theca on the one 

 side, and the right spinose corner of the theca on the opposite margin lie 

 on the margin of the fossil, and the flattened thecal edge looks almost 

 straight. This is one of the most frequent and easily understood appear- 

 ances. We have a fossil which while it reminds us of D i p 1 o . quad r i- 

 mucronattis is nevertheless easily separable. 



(r) We may have an angle of compression intermediate between « and ,'-. 

 Some of the spines may show, but being cut at angle, only very inconspicu- 

 ously. The "welt" however becomes in this case very conspicuous and 

 the apertural margin of each theca becomes broadly concave. 



The scalariform views are simpler and easy of interpretation. 



It is not at all unlikely that the characters we regard here as specific, 

 are actually subgeneric, but as yet our evidences are imperfect, and will 

 probably remain so until we can study many specimens of the types and 

 varieties in relief. 



In the meantime it is safest to regard the several recognizable subforms 

 as varieties of D . foliaceus. 



One of the varieties occurring in the Stockport shales is identical with 

 my var. acutus from S. Scotland, Shropshire, Conway ? and Lower 



