MorjjhoJogij of the Rhahdophora. bl 



dividing ridge, but the ridge itself, pvojecting into the internal 

 cavity of the graptolite, and so clearly in some instances that 

 when examined under the microscope its thickness can be mea- 

 sured and the extent of its projection estimated. The accom- 

 panying figures (figs. 2, 3) are reduced from drawings thus 

 made with the microscope and camera lucida. 



It would thus appear that in certain graptolites the calycles 

 seem to be completely cut off from their supporting perisai'c, 

 this appearance being due to a constriction or the presence 

 of a partially-dividing ridge, and also that in these same 

 forms there are at least constrictions in the perisarc dividing 

 it into sections, from each of which a calycle is produced. 

 This is the structure which generally obtains in the recent 

 Thecaphora ; and I therefore think that it can now no longer be 

 maintained that the calycles of the graptolite are not true 

 hydrothecse, — the conclusion arrived at from previous investi- 

 gations into the morphology of the Riiabdophora, and espe- 

 cially of the reproductive organs of certain graptolites *, that 

 they are the Pahcozoic representatives of the recent Hydro- 

 phora, thus being confirmed by specimens from rocks which 

 would naturally be supposed to be most unlikely to yield 

 fossils showing minute internal structure. 



That these appearances have not been more frequently seen 

 is probably owing to the imperfect state of preservation in 

 which the Khabdophora usually occur, and the very rare 

 occurrence of specimens in section with the interior removed. 



Mr. Dover's collection of graptolites is probably the most 

 complete which has hitherto been made from the Skiddaw 

 Slates ; and a careful examination of it might add considerably 

 to the hitherto-known fauna of these beds. Some graptolites 

 are shown by specimens in his possession to have attained a 

 very large size, there being many single branches of Didymo- 

 grapti and 'retragrapti about a foot in length, a few of which 

 show no signs of termination at either end. Every division 

 of the series has been diligently worked for fossils by him ; 

 but it is only from one bed, exposed at Falcon Crag, that 

 specimens preserved in an uncompressed state and showing 

 internal structure have been obtained. 



* See Ann, & Mag. Nat. Hist. (ser. 4) vol. vii. p. ol7. 



