C. Lapwort/i — On British Graptolites. 557 



Orthograptiis,' ■which have as yet offered no trace of a true sicula, 

 belong in reality to the GlossograptidcB. 



The genus Diplograptus is here divided into several sub-genera. 

 This will be found of service in enabling us to group together, 

 for convenience of reference, species having a certain amount of 

 similarity in their external features. It will also have the further 

 advantage of necessitating such close attention to those minor, and 

 hitherto neglected points in outward form dependent upon internal 

 structure, as may result in the detection of the true relationships of 

 the very different fossils which are at present associated in this 

 badly understood genus ; and ultimately furnish us with the means 

 of accurately defining the exact line of separation between the two 

 great sections of the Khabdophora. 



The distinguishing characteristics of these sub-genera will, it is 

 hoped, be sufficiently apparent from the analysis in the Table, and 

 from the references to the typical species. 



Eegarding the controverted question of the relationships which 

 exist between the DiiplograptidcB and the genus Betiolites and its 

 allies, it must be admitted that, trenchant as are the apparent dis- 

 tinctions in mode of development and general plan of structure 

 which characterize these two sections, yet every new investigation 

 results in the detection of fresh and striking correspondences in 

 detail of form or structure. A full consideration of these numerous 

 points of contact may here be dispensed with. It will be sufficient 

 for the present merely to call attention to the following facts. 



While in some Eetioloid species the virgula is either entirely 

 wanting, or is represented by a slender thread forming a portion of 

 the external network, in others it appears to be a stout double rod, 

 comparable in all respects with the virgula of the duplicate Grapto- 

 lites. 



Some siculate Graptolites have the outer angles of their thecee in 

 part strengthened by a chitinous thread, a structural feature especially 

 characteristic of the BetioUtidce. 



In some forms of the Diplograptidce the polypary on its obverse 

 aspect presents us with a well-defined suture, while on its reverse 

 aspect the periderm offers no trace whatever of a central groove, but 

 is entire and unbroken ; leading us to conclude, that in these abnor- 

 mal specimens the common canal was only partially divided : thus in 

 effect communicating with both rows of thecal as in Betiolites. 



On the other hand, in some true Eetioloid species, the two lateral 

 halves of the polypary fall asunder after death, remaining attached 

 by their proximal ends only : thus presenting us with a form which, 

 excepting in its reticulate structure, irresistibly reminds us of 

 Dicranograptus, a genus notably illustrative of the duplicate nature 

 of the polypary in the Diplograptidce. 



For these and many other reasons, some of which will become 

 apparent on a study of the Table, it is evident tliat the great groups 

 of the siculate and non-siculate Bhabdophora afford so many minor 

 structural correspondences that no scheme of classification havino- 

 any pretence of representing, even approximately, the order of 



