STUDY OF THE GRAPTOLITES. 229 



mucronate extension, and apparently simulate the more common forms 

 of Graptolites (Plate i, fig. 5). 



In the bicelluliferous species the compressed specimens present the 

 plane of the orifice, sometimes rectangular to the general axis, sometimes 

 with the outer margin a little advanced , making an acute angle with the 

 axis of the cellule ; while sometimes the plane of the margin of the aper- 

 ture is rectangular to the axis of the cellule, or rarely makes with it an 

 obtuse angle. The cellules of Retiograptus, which have not yet afforded 

 means of satisfactory examination, apparently have their orifices nearly 

 rectangular to the general axis of the stipe. 



In one of the forms of the bicelluliferous Graptolites, the cellules are 

 sub-elliptical tubes, with an orifice of corresponding form, without exten- 

 sion beyond a slight thickening or callosity at the margin of the orifice. 

 The plane of the cell-aperture in this one makes an obtuse angle with 

 the direction of the general axis. 



In forms like G. bicomis, the external orifice is transversely oval, with 

 or without a projection and thickening of the test from the cell-partition 

 above the orifice, or extension of the test. 



4. OHnsrAMENTS OF THE TEST. 



The compressed condition in which the Graptolites usually occur, is 

 unfavorable to the preservation of any minute surface-markings, or orna- 

 ments of the test. 



In many of the species, fine striae, parallel to the margins of the cell- 

 apertures, are perceptible, and in the larger number of species this 

 marking is all that is preserved. There is sometimes a granular 

 appearance of the surface ; but I have not been able to satisfy myself 

 that this is the actual surface-texture, and it may be a condition induced 

 by mineralization. In a few examples, there is a row of minute pustules 

 at the base of, and corresponding to the cellules. 



The stems and branches of Dendrograptus, Callograptus and Dictyo- 

 NEMA, are irregularly striated. In typical species of Retiolites the test 

 is finely reticulate; while in the species from the Quebec group, this 

 texture, if existing, is so fine as not to be readily resolved by an ordinary 

 lens. The surface, however, has not the appearance of entire smooth- 

 ness, as in most of the ordinary Graptolites. 



The chief ornaments of these bodies are the mucronate extensions of 

 the test, usually from the lower margins of the cellules, but sometimes 

 from the upper margins. In ordinary forms of the species, with single, 



