TETRAGRAPTUS. 55 



Genus TETRAGRAPTUS, N' J/ r. 



863. Tcf ragrapsns, Siiltcr, "Note ou the Skiddaw Slate Fossils," Quart. Jouru. Geol. Soc, vol. xix, 

 p. 137. 



J-'ol i/J>'ir// hilateraWy Hymrnvtvica], consistino- of four uuiscrial main stipes, Avliieli 

 are produced l)y dicliotouious division from a, primitive " J)i(li/iii(i</nijitns 

 sl(i(je ; " these stipes may grow upward, outward, or downward. 

 Tliccx. — Sim})le cylindrical or subcylindrical tubes expanding slightly in the 

 direction of their apertures, inclination variable, usually in contact for a 

 considerable portion of their length. 



The polypary is characterised by the i)ossession of stii)es of two orders oidy. 

 Those of the first order are short, and constitute together in this genus the 

 " funicle " of the earlier authors ; those of the second order are long, and form the 

 four main stipes of the polypary. These four main stipes may ultimately grow 

 in different directions in the several species, but they invariably originate in a 

 primitive iJulyviograptus stage, which is usually horizontal. 



The earliest tlieca develops fi'om the sicula in a manner precisely similar to 

 that described for BiihjDiogro^jfus {anfc, pp. 6, 7). It may originate (a) near the 

 apex of the sicula {T. Bigshgi), or (h) close to the apertin^e itself ('/'. quadribra- 

 ckiatiis). The crossing canal and second theca are also developed on the Didgino- 

 grajdus plan. After the development of one or more thecEe, however, a dividing 

 wall (see Fig. 40 a) makes its appearance in each young stipe ; and hence from one 

 theca on each stipe two theca3 are developed. These grow apart from each other, 

 and originate two stipes on each side of the sicula, instead of one only, as is the 

 case in DldAjmograptiis. 



The Didijmograptus stage is invariably short-lived, but it seems to persist longer 

 in some forms of Tetragraptus than in others. In T. Bigshyi, for example, as Holm 

 has shown (' Geol. Foren. Forhandl.,' vol. xvii, p. ol9), the dichotomous division takes 

 place in the two thecal which are first developed (tli. V and th. 1-) ; and thus the 

 two stipes of the first order are reduced to a mininmm as regards their length; 

 while in such a form as T. qiiadribnichiatus the earliest thecee remain undivided, 

 and the second or third thecas on each side of the sicula (th. 2^ and 2- or 3^ and 3") 

 undergo division ; so that the stipes of the first order in the former case consist each 

 of one theca, and in the latter case of two or of three thecse. After division the 

 development and growth of the four main stipes proceed in the normal manner of 

 those of Didijnwgraptas. 



The Tetragrapti, like the J lidi/iiKigntjili, fall into natural sei'ial groups, 

 according to the ultimate direction of their stipes; thus we have a horizontal 



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