248 



BRITISH GRAPTOLITES. 



Fio. 167 



a — d. — Glyptograptus tama- 

 riscus, Nieh. 





5S 







and outward ; tli. I 3 grows from it obliquely across the back of the sicula, so as 

 to leave it free on one side for a considerable fraction of its length; its 



aperture is on a level with the apex of the sicula. 

 The septum commences at once, and is continued 

 throughout the whole obverse aspect of the polypary, 

 but is obviously incomplete, since there is no trace 

 of it whatever in the reverse aspect. 



The thecos are distinctly alternate in their 

 arrangement, and frequently show well-marked 

 growth-lines. They are rather more than 1 mm. in 

 length, and overlap about one quarter to one third of 

 their length. In the bi-profile view the apertural 

 margins appear horizontal or slightly introverted; 

 they are situated within long and deep excavations, 

 occupying one half to one third of the total breadth 

 of the polypary. But while the ventral wall of the 

 theca is distinctly sigmoid, it is more flowing than 

 that of species belonging to the genus Climaco- 

 graptus, and the rounding of the free edge is 

 particularly clear. 



Affinities. — G. tamariscus is distinguished from 

 other species of Glyptogr aphis by : (1) the smaller 

 amount of overlap of the theca ; (2) the more flowing outline of the ventral wall ; 

 (3) the greater depth of the excavation. 



Horizons and Localities. — Llandovery, Birkhill Shales, Skelgill Beds (zones of 

 Monog. gregarius and M. spinigerus and their equivalents). 



Lalee District : Skelgill ; Browgill ; Troutbeck Valley, etc. 8. Scotland : Dobb's 

 Linn; Belcraig ; Garple Linn; Mount Benger Burn; "Woodlands, Girvan, etc.; 

 Wales : Rhayader ; Llanbrynmair. Ireland : Donaghadee, Co. Down ; Little River, 

 Pomeroy, Co. Tyrone. 



Associates, etc. — Glyptog. tamariscus occurs plentifully in an excellent state of 

 preservation in the Skelgill Beds of the Lake District ; it is also abundant, but in 

 a compressed condition, in the Birkhill Shales of S. Scotland ; its common 

 associates are Monog. gregarius, M. triangularis, CI. Tornguisti, and CI. rectangularis, 

 at the lower horizon, and Monog. spinigerus, M. argenteus, M. Cling ani, M. lobiferus, 

 and CI. scalaris, in the higher beds. 



Collections. — British Museum (Natural History), Sedgwick Museum, Dr. Marr, 

 Lap worth, and the Authors. 



a. Small specimen in relief, obverse 



aspect, showing the sicula and septal 

 groove. Ambleside, Skelgill Shales. 

 One of Nicholson's original speci- 

 mens. Brit. Museum (Nat. Hist.). 



b. Reverse aspect of proximal end of 



specimen in relief. Note absence 

 of septal groove. Ibid. 



c. Distal thecse of same specimen. 



d. Distal thecse of wider compressed 



specimen, showing greater amount 

 of overlap. Enlargement of part 

 of PL XXX, fig. 8 b. 



