Vol. 54.] 



FAUNA OE THE SKIDDATV" SLATES. 



51" 



Genus Diplograptus, M*^Coy. 



(a) DiPLOGHAPTUS DEISTTATTJS (BrOHgll.). 



1828. Fucoides dentatus, Brongu. ' Hist. Veget. Foss.' vol. i, p. 70 & pi. vi, figs. 9-12. 



1865. Diplograptus prist iniformis, Hall, ' Grajit. of the Quebec Group,' Geol. 

 Surv. Canada, dec. 2, p. 110 & pi. xiii, figs. 15-17. 



1868. Diplograpsus pristiniformis, Nich. Quart. Journ. Geol. Soc. vol. xxiv, p. 140, 

 pi. V, figs. 14 & 15. 



1875. Diplograptus dentatus, Hopk. & Lapw. ihid. vol. xxxi, p. 656 & pi. xxxiv, 

 figs. 5 a-k. 



The species pristimformis .seems to be the same as the form 

 described earlier by Brongniart as Fucoides dentatus, but I do not 

 agree with other authors who consider it identical with Hisinger's 

 species teretiusculus ; the latter has fewer thecae in the same unit of 

 length, and these are inclined at a higher angle. 



The species varies greatly in its dimensions, and the appearance 

 of the thecae is so different in various aspects of the rhabdosoma, 

 that it is only after examining a great number of specimens, and com- 

 paring them most carefully with each other, that I have come to the 

 conclusion that these different appearances are presented by the same 

 species. In one aspect the rhabdosoma resembles very closely some 

 young forms of D. foliaceus (Murch.), in another it appears to be 

 identical with some forms of Olimacograjptus^ while in a third the 

 apertural margins of the thecae are deeply concave and their outer 

 edges broadly rounded. 



The variation in dimensions and number of the thecae is shown 

 in the following table : — 



Form and Locality 



Thornship Beck, (a) 



Aik Beck, Pooley. (a^) 



}> >j >j (^ / 



Troutbeck 



Ellergill, Milburn. (a^) 



N.W. Longside, Skiddaw. 



Mungrisedale 



Bassenthwaite Sand-beds. 



Length 



1 in. (254 mm.) 



incomplete 

 ^ in. (6'3 mm.) 

 lin. (254 mm.) 

 yV in. (14-7 mm.) 

 1 in. (254 mm.) 

 1^ in. (31-7 mm.) 

 ^in. (12-7 mm.) 

 1 in. (254 mm.) 

 l^in. (38 mm.) 



f in. (19 mm.) 

 1 in. (254 mm.) 



Maximum 

 width 



-Vin 



-S in 

 V in 



(2*1 mm.) 

 (r58mm. 

 (r58 mm. 

 (24 mm.) 

 (1-58 mm. 

 (1-58 mm. 

 (158 mm. 

 (1-58 mm. 

 (24 mm.) 

 (l"58mm. 

 (24 mm.) 

 (l'58mm. 

 (r58mm. 



No. of 



No. of 



thecse in 



thecae in 



1 inch 



10 mm. 



30 



12 



28 



11 



28 



11 



36 



14 



36 



14 



32 



13 



32 



13 



32 



13 



24 



9-10 



28-32 



11-13 



28 



11 



28 



11 



28 



11 



As will be seen from the above table, the dimensions given, by 

 Nicholson are often exceeded, and there is great variation from the 

 number of thecae stated by Hall. The highest number of thecae 

 generally includes those of the proximal end, which are more closely 

 set than those in the adult part of the rhabdosoma. 



The maximum width of the rhabdosoma is always reached at an 

 early period of its growth, and is then maintained for the remainder 

 of its length. The proximal end of the rhabdosoma is seen to be 



