GRAPTOLITES OF NEW YORK. PART 1 



555 



and compared the thecal characters, and the measurements of the number 

 of thecae within a certain space, of their inclination, apertural angle and 

 ovei'lap, and by means of these exact data confirmed the close resemblance of 

 the characters of the thecae in members of different genera. 



Miss Elles's investigations have led her to agree with Nicholson and 

 Marr (1) that these resemblances are of genetic origin, and therefore (2) of 

 systematic value ; and further (3) that in any natural group the forms with 

 relatively fewer branches were developed from the more complex forms ; so 

 that it follows from this (4) that the so called " genera " of the usually 

 accepted classification of the Dichograptidae are far more of a chronologic 

 than of a zoologic significance. Her own work has suggested the further 

 conclusion that the forms in question are most probably the result of develop- 

 ment along certain special lines. 



Altogether eight groups with like thecal characters are recognized by 

 Miss Elles among the Dichograptidae of the Skiddaw fauna. • Some of these 

 groups (five) are derived from the early multiramous genus Bryograptus, and 

 the remaining three from Clonograptus. In each case the reduction in the 

 powers of dichotomy leads first to a Tetragraptus and finally to a Didymo- 

 graptus. As many of the forms are also represented in our Deep kill fauna, 

 Ave cite these groups and invite the reader to test the similarity of 

 the forms belonging to the same groups, as far as possible by using the 

 plates. 



(1) GRAPTOLITES DERIVED FROM BRYOGRAPTtTS 



(a) Bryograptus ramosus var. cumbrensis 



Tetragraptus pendens 



* Didymograptns indentus 

 {h) Bryograptus ramosus var. cumbrensis 



Tetragraptus fruticosus 



Didymograptus furcillatus 



(c) Bryograptus ramosus var. cumbrensis 

 Tetragraptus postletLwaitii 

 Didymograptus hifidus 



(d) Tetragraptus higsbyi 



T. phyllograptoides 

 Phyllograptus angustifolius f 

 Didymograptus giliberulus 

 ( e ) Bryograptus callavei 



Tetragraptus sp. (Hincksii type) 

 Didymograptus nicholsoni 

 Azygograptus lapworthi 

 Didymograptus affinis 

 Azygograptus suecicus 

 Didymograptus gracilis 



