154 



CHAPTER VII.* 

 SILURIAN RADIATA. 



ZOOPHYTA. 



Fam. GRAPTOLiTTDiE (M^Coy). 

 Stem simple or branched, thin, usually linear, horny, unrooted ; 

 polype-cells sessile in one or two rows; each cell divided at 

 bottom by a transverse diaphragm. 



In the form of the thin horny polypidom and polype-cells the 

 Graptolitidce agree with the Sertulariadcej but diflfer in not being 

 rooted. I have never found any trace of ovarian vesicles ; others 

 may find them, hov/ever, by examining great numbers of speci- 

 mens, or by some fortunate accident ; or, like the closely allied 

 Corijmorpha (which agrees with the Graptolites in having a free 

 polypidom), the ova may have been developed in naked sacs at- 

 tached to the base of the tentacles of the polypes, and would not 

 leave any trace in the fossil state. 



On the same grounds that the allied recent genera Plumularia 

 and Sei'tularia are separated, I propose to restrict the term Grap- 

 tolites to those which, like the original typical species, have the 

 cell-denticles only on one side, and for those having them on 

 both sides I propose the generic name Diplograpsus. Prof. Nilsson 

 and Col. Portlock have published nearly similar views, the latter 

 with great clearness. 



Graptolites lohiferus (M'Coy). 

 Sp. Char. Several inches long, straight ; axis capillary, undivided 

 portion of the stem about one-third of a line wide ; cells form- 

 ing large, obtusely rounded lobes, uniformly convex on the 

 upper and outer margin ; a notch on the outer edge separates 

 the rounded extremity from the oblique descending margin ; 

 four cell-lobes in 3 lines ; wudth from axis to end of cell-lobe 

 slightly less than 1 line. 



The rounded lobe-like form of the cells and their size render 

 this one of the most distinctly marked species of the genus. 

 Fragments are often beautifully iridescent. 



Common in the black shale of Lockerby. 



[Col. University of Cambridge.) 



* The species in this chapter date from the ' Annals ' for October 1850, 

 except Strephodes trochiformis, S. pseudo-ceratites, Ccenites strigatus and 

 PalcEopora subtilis from the Number for December 1850. 



