STUDY OF THE 0RAPT0LITE8. 249 



by slight oval pustules, or oval depressions, upon the extremely com- 

 pressed surface of the stipe ; but it cannot be satisfactorily shown that 

 this appearance indicates the normal condition of the cellule or the 

 aperture. If the true form be in reality so far different from the usual 

 character of the Graptolitidse as these appearances indicate, it may be 

 found necessary to separate them from this family. 



The Genus Dictyonema is restricted to such forms as have the numerous 

 stipes and branches connected by transverse processes, and the whole 

 united in a flabelliform or funnel-shaped frond, without elongate stem or 

 trunk. The stipes and branches are irregularly striated externally, con- 

 sisting of a corneous envelope, as in ordinary graptolites ; but I have not 

 been able to determine clearly the existence of a solid axis. The cellules 

 are indicated by angular processes or cell-denticles on the inner side of 

 the branches, as shown in fig. 5 of Plate i. 



In the Genus Rastrites of Barrandb, the distinguishing features are 

 the slender cyliudrical stipes or branches, with slender tubular cellules 

 which are free throughout their entire extent. 



The few species of Thamnograptus known, consist of cjdindrical or sub- 

 cylindrical stipes, with slender elongate alternating pinnulae or branchlets. 

 No evidence of cellules has been observed in any of the specimens.* 



The peculiar forms for which I have proposed the name Ptilograptus, 

 consist of branching stems, which, in all their divisions, are studded on 

 each side, in alternating order, with narrow pinnulae. These are some- 

 times extremely slender, or even capillary in their dimensions. In one 

 species I have detected elliptical spots upon one face of the pinnulse 

 which are slightly flattened, and I infer that these are the cell-apertures. 

 The substance of the test is corneous, and there is an internal solid axis. 

 Although I have placed these forms under the Graptolitidae with some 

 hesitation, the form of cellules may perhaps render a separation desira- 

 ble ; but with our present information, such a separation cannot at this 

 time be made. 



The Genus Inocaulis was proposed for some flattened stipes with a 

 scabrous surface, which have the appearance of denticles upon the mar- 

 gins. These stipes grow in close groups or tufts, and are bifurcating or 

 branched in their upper portions. No positive evidence of cellules has 

 been observed. The presence of denticles, together with a corneous or 



*It is possible that this genus may have more intimate relations with Rastrites than would 

 appear from the species now known. 



Cab. Nat. 32 



