128 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



knowledge, are the possession of a frond composed of a variable 

 number (always more than four) of simple stipes, united centrally at 

 the base by a non-celluliferous stem or " funicle." The primary 

 subdivisions of the funicle are always, and the secondary subdivisions 

 generally, non-celluliferous (Dichograpsus Milesi, Hall, sp., being 

 the only form in which the reverse is certainly the case). The 

 stipes are monoprionidian, and are given off from the funicle in a 

 radiating manner. The divisions of the funicle are sometimes, but 

 not always, enveloped by a corneous disk. 



In the typical species of the genus, namely D. Logani, Hall, sp., 

 and D. ocioh^achiatus, Hall, sp., the ceUuhferous stipes are never 

 divided, but the frond becomes compound simply by the branch- 

 ing of the funicle ; in these also the central disk is usually present, 

 though not uniformly so. In another group, of almost subgeneric 

 value, comprising D.j/?g.i^i7i5, HaU, Z>. ingidus, Hall, D. multiplex, 

 Nich., and other species, the celluliferous branches are themselves sub- 

 divided, so that the frond becomes doubly compound, whilst the 

 central disk appears to have been constantly absent. The fact that 

 the central disk is present in some BicJiograpsi and wanting in 

 others, not being uniformly present in different individuals of the 

 same species even, whilst it occurs also in some species of Tetra- 

 grapsus, is sufficient to show that it cannot be considered a generic 

 character. It appears to be composed of two corneous laminae, 

 united at their edges, but enclosing a central cavity ; and it is 

 believed by Prof. Huxley and Mr. Salter to be most closely analogous 

 to the basal plate of Defranc'ia, a Polyzoon. I am, however, inclined 

 to believe that a much more feasible homologue is to be found in the 

 " float" or " pneumatophore " of the PJiysopTioridce, an order of the 

 Oceanic Hydrozoa. This view is rendered more probable by the 

 occurrence of a somewhat similar disk in some specimens of JDiplo- 

 grapsus hicornis, Hall, as- originally pointed out by Hall ; and I have 

 observed the same phenomenon in a species of Diplograpsus allied 

 to D. palmeus, Barr., from the GraptoHtiferous rocks of Dumfries- 

 shire. If, however, this conjecture be correct, we should have to 

 believe that this organ was developed only at certain stages of 

 growth, in certain individuals of the species, and probably for 

 certain definite purposes. Indeed, in Diplograpsus hicornis, Hall, 

 an unbroken series of gradations can be traced, from those indi- 

 viduals in which the disk is whoUy wanting, up to others, in which 

 it is largely developed. 



The species of Dichograpsus known to me as occurring in the 

 Skiddaw Slates are three in number, namely D. Logani, Hall, D. 

 octohracJiiatus, Hall (=D. Sedgwickii, Salt.), and D. multiplex, 

 Mch., of which the last is new, whilst the other two are weU- 

 known Canadian forms. 



1. DiCHOGKAPSTJS LOGANI, Hall, Sp. 



Graptoliihus Logani, Hall (Pal. N. York, vol. iii., and Grapto- 

 lites of the Quebec group, p. 100, pi. ix.). 



This is a very beautiful and characteristic Canadian species, of 



