GEOLOGY AND IIINEEALOGT. 



3S9 



skeleton ; and occur either free, or as an incrustation on stones, shells, sea weeds, 

 and other bodies. The common form of the graptolite is that of an extremely 

 narrow band, dentated or toothed on one or on both of its edges. The so called 

 teeth constitute the cells or dwelling chambers of tl ■ separata animals (or vital 

 centres) of the organized mass. The baud itself, or 6 ape, is occasionally expanded 

 into a leaf-like form; or narrowed, on the other hand, into a mere thread. Many 

 specimens exhibit also a filiform central axis, often extending beyond the stipe. 



These fossils have been grouped in several genera, of which the more important 

 comprise: — G raptolithus (teeth on one side only, but close together, stipe un- 

 branched). Hastritcs (teeth far apart; stipe unbranched, but often convoluted). 

 Didymograpsus (teeth on one side ; stipe branched). Diplograpsus (teeth on 

 each side of stipe). Certain branched forms, however, have a single row of teeth 

 on the branches, whilst the main stem is toothed on both sides : and hence, in 

 cases of this kind, fragments of one and the same form might be referred to dif- 

 ferent genera. The deeply recurved species of Didymograpsus, a<rain, constitute 

 transitional forms between that genus and Diplograpsus. The non-occurrence of 

 doubly-toothed forms, as pointed out by Sir Roderick Murchison, in the Upper Si- 

 lurians of England, does not militate against these objections : because, (so far as 

 regards species) graptolites are never abundant in these upper rocks, and doubly- 

 toothed species are found in them in other localities. There can be no doubt, 

 moreover, as shewn by Sir W. Logan's discovery in the Point Levi district, that 

 our common specimens exhibit merely a fragmentary condition of the original 

 graptolite structure. The genus Rastrites does not appear to have been discover- 

 ed in America. The other genera are included in this note, under the common 

 term of G.-aptolithus. 



§ 1. Graptolites with murronate serratures. 



G. mucronatus, Hall. Unbranched ; flattened ; doubly-toothed ; teeth with 

 mucronate and more or less wavy tips. Fig. 1. Hudson River group : Albany. 



G. sextans, Hall. Branched ; the branches separating from the base, and ex- 

 hibiting teeth on the outside only ; tips delicately mucronate, but often badly pre- 

 served. Fig. 2. Hudson River group. 



