STUDY OF THE GRAPTOLITES. 215 



The species of Retiog7'aptus, while having some characters in common 

 with Retiolites, do not possess the reticulated structure of the test in either 

 of the described species. There yet remains some obscurity in regard to 

 the internal structure of this genus, which can only be satisfactorily 

 explained by the examination of better preserved specimens. Neverthe- 

 less, in its general form, structure and mode of growth, it is shown to be 

 quite distinct from other graptolitic genera. The three species referred 

 to this genus present differences which can only be reconciled by supposing 

 that the two sides of the stipe are very unlike each other in form and 

 external characters, as in Retiolites. 



The species for which I have proposed the generic name Phyllograptvs^ 

 present close analogies with the typical form of Diplograptus in the character 

 of test and form of cellules. These, in their aperture and form, are nearly 

 quadrangular ; and the cell-partitions are apparently continuous between 

 the cellules, and reach nearly to the central axis; characters which we find 

 in Diplograptus These forms, in their great lateral extension, depart 

 widely from their analogues ; but they differ more essentially in their 

 cruciform mode of growth, presenting an arrangement of parts, as if four 

 simple stipes (like those of G. bryonoides or G. bigsbyi) were joined 

 together by the coalescing of the solid axes. In this latter respect, and 

 in their great development in width, they differ most essentially from all 

 the other genera of this family of fossils. These forms are illustrated on 

 Plate iv of this memoir. 



In the typical species of Dendrograptus, as illustrated under the generic 

 description, and in some other species, we have a wide departure from 

 the typical forms of Graptolithus, as developed in the characteristic 

 species of the genus. The strong stem or trunk, which is free from 

 cellules, and which has apparently been fixed at the base; the irregular 

 branching, which has no bilateral, and apparently no definite arrange- 

 ment, such as observed in all the forms of true Graptolithus, are strong 

 points of dissimilarity, and furnish characters for generic distinction. 

 The stem and branches are unequally striated longitudinally, but the 

 form of the celluliferous branches and of the cellules offers no important 

 difference (except in the smaller dimensions) from those of the stipes or 

 branches of the usual form of graptolites with a single series of cellules. 

 In one species referred to this genus (the D. gracilis) there is some 

 departure from the typical form of cellules, and the body of the stipe is 

 contracted at intervals, while the form of cellule and cell-aperture is 

 not unlike some of the sertularians. 



The Genus Callograptus offers forms which are intermediate between 

 true Dendrograptus and Dictyonema. In these species, the form of tlie 

 cellules has not been fully determined. They are marked in one species 

 by slight oval pustules, or oval depressions, upon the extremely compressed 

 surface of the stipe ; but it cannot be satisfactorily shown that this appear- 

 ance 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 



