538 Trof. Nicholson 8f J. E. Marr — Phylogeny of the Graptolites. 



may be more remote than those to members of some other family. 

 Thus, Hall (Grapt. Quebec Group, pi. xvi, fig. 26) figures a form 

 of Tetragraptus which he refers to T. Bigsbyi,^ in which the apices 

 of the stipes are in contact, and remarks that "it is difficult to 

 separate such forms from Phyllograptus." In fact, having examined 

 many specimens, we are disposed to suggest that this form is truly 

 intermediate between Tetragrapti of the Bigshyi type and Phyllo- 

 graptus. 



Furthermore, as the unilateral Monograpti are related to the 

 bilateral Diplograpti through Dimorphograptus, the same reasoning 

 applies to these ; and the single genus Monograptus may contain 

 descendants of more than one " family." This would account for 

 the great diversity in the character of the Monograptid hydrothecse. 

 Many Monograpti possess hydrothecse resembling those of pre- 

 existing Diplograptidae, whilst one well-known species, M. argutus, 

 has hydrothecae closely similar to those of the family Dicrano- 

 graptidse. 



Should our conclusions be correct, the present nomenclature will 

 have to be altered, for it is clear that two species of Didi/mograptus, 

 to take an example, which have descended through different Tetra- 

 graptid forms, the one from a Bryograptns, the other from a 

 Dichograptus, cannot be relegated to the same genus. In the present 

 state of our knowledge we believe it would be not only inconvenient 

 but also unpardonable to alter a classification the working value of 

 which has been made fully apparent, and we propose, therefore, to 

 retain such names as Monograptus, Didymograptvs, and Tetragraptus 

 as " generic " names, merely observing that we do not consider the 

 species placed under these various groups to belong to definite 

 genera, in the sense in which the word is used by biologists, but 

 that they constitute cases of what Buckman terms heterogenetic 

 homoemorphy of forms which are only distantly allied to one another. 



To a considerable extent these homoeomorphic forms are iso- 

 chronous and not heterochronous, as proved by detailed work in 

 the field ; hence their extreme value to the stratigraphical geologist. 

 They may be regarded as constituting a special case of mimicry, the 

 reasons for the existence of which may be very briefly discussed. 



We are indebted to Mr. Clement Reid for the first suggestion 

 that the variations in the characters of Graptolites may be connected 

 with the supply of food, and study of the variations inclines us to 

 accept this suggestion as plausible, if not correct. 



The earlier Graptolites (such as Dictyograptus in the Lingula flags, 

 etc.) consisted of a number of irregularly-branching stipes ; but 

 symmetry in the arrangement of the stipes would tend to ensure 

 an equal supply of food to each stipe ; hence we soon meet (in the 



^ We have examined a large number of examples of the Tetragraptus here alluded 

 to, which have been collected in the Skiddaw Slates of Troutbeck and Outerside, 

 near Keswick, and are of opinion that it is specifically separable from T. Bigsbyi, 

 Hall, and ought to receive a new name. Pending a complete description, we may 

 therefore speak of this form as T. inosculans. In some of our specimens of this 

 singular form not only are the apices of the stipes in contact, but more or less 

 complete fusion or cohesion has taken place. 



