GRAPTOLITES OF NEW .YORK, PART 2 125 



ograptus rest. Since also the thecae of C. retioloides and Lasi- 

 ograptus are remarkably similar there is little doubt that the former points 

 out the path of development of Lasiograptus. If, however, this view of 

 the origin of Lasiograptus is correct, it is evident that the reticulate outer 

 structures of the thecae of that genus are of secondary origin. The 

 peculiar contraction of the triangular appendages towards their base is also 

 quite significant in this connection since it shows that these appendages 

 which separate the distal parts of the zooids are not merely prolongations 

 of the thecal walls but new formations, starting from the bases of the 

 loops of C 1 i m a c o g r a p t u s retioloides. 



We have in the foregoing discussion of Lasiograptus contrasted the 

 outer reticulate structure of the thecae with the solid periderm of the latter. 

 There is, however, good reason to assume that this continuous periderm 

 partakes also of the nature of the reticulate walls of the Retioliticiae but is 

 covered with a thin continuous layer similarly as in Retiolites. The main 

 reason for this view is that the rhabdosome of Lasiograptus never possesses 

 the strong gloss of the solid walled forms as Dicranograptus and Diplo- 

 graptus but is always of a dull black color and gives the impression of 

 original tenuity. Furthermore we have observed in somewhat macerated 

 specimens indications of the presence of a system of stronger marginal 

 ledges recalling" the structures found in other Retiolitidae [see pi. 31, fig. 8 J. 

 The latter observation is also to be considered as a further argument for 

 the completeness of the proximal parts of the thecae and the secondary 

 origan of the outer structure. 



A like dullness of the periderm indicating great tenuity of the walls is 

 a characteristic feature of the species of Cryptograptus which as well as the 

 punctate nature of the test have already been pointed out in Lapworth's 

 diagnosis of the genus. We have material in hand, which demonstrates 

 the presence of stout apertural rings of the sicula and a system of marginal 

 ledges of the thecae [see postea\. The combination of the punctate and 

 much attenuated test with the presence of these ledges leaves hardly any 

 doubt that this well defined group of forms has already reached the retio- 



