64 



[Senate 



GENUS RASTRITES (Barrande). 

 This genus was proposed by M. J. Barrande in 1850, to include certain grapto- 

 litic forms which he describes as follows : 



" This polypier is composed of a small, almost linear, very long, and slightly 

 " curved stem, provided with an interior canal forming the communication between 

 " all the cellules. These are disposed upon the convex sides of the axis, and make 

 " with it a slightly acute angle : they are completely isolated from each other. The 

 " proportion between their length and their reciprocal distance varies according to 

 *' the species. The diameter, in the known species, is always greater than that of 

 " the stem to which they are fixed. 



" Distribution of species. The Genus Rastrites has hitherto been represented 

 " only by four forms, all belonging to Bohemia : they characterize the mass of the 

 " Graptolite schists, constituting the base of our upper division. One of these four 

 *' forms, Rastrites peregrinus, is also found in Saxony. 



" Relations and differences. It must be observed that there is a very great ana- 

 " logy between the Genus Rastrites and the Subgenus Monoprion. The only 

 " characters which lead us to separate them are : 1, The isolation and the great 

 " space between the cells composing the polypier, which we call Rastrites; 2, The 

 *' great tenuity of their filiform stems, always more slender than the alveoles 

 ** which they support." 



Notwithstanding some slight differences from the generic description here given, 

 I have referred the following form to this genus : 



RASTRITES BARRANDI ( n.s.). 



Stipe slender, filiform, rigid, slightly curved, and furnished on the concave side 

 with numerous, nearly regularly disposed, minute, setiform processes or cellules, 

 at the bases of which there is a slight thickening or expansion of the principal 

 stipe. Stipe, and cellules or processes, rounded in their natural condition. 



Rastrites barrandi. 



The fragment is about two and a quarter inches in length, 



and in its natural state has evidently been a nearly or quite 

 cylindrical tube, a longitudinal depressed line indicating 

 the place of the axis. In this length there are more than 

 forty of these minute processes, the stipe just below each 

 one swelling out a little on that side; the expansion ter- 

 minating abruptly above, and, from its outer angle, the 

 minute spine proceeds like the mucronate extension from 

 the points of the serratures in some graptolites. 



This form differs from the strict description of Rastri- 

 tes, in having the stipe much larger than the cellules. 

 Whether these points or processes are the true cellules, or 

 extensions from them, might admit of some doubt, were 

 observations based upon this species alone. 

 1 3 Fig. 1. View of the specimen, nat. size. Fig. 2. A part enlarged. 



Geological position and locality. In the shales of the Hudson-river group : 

 Near Albany. Collected by Mr. R. P. Whitfield. 



