240 



IMilTlSIl GRAPTOLITES. 



I<'[<is. L59 a and b. Orthograptus calcaratus, TiAyw, 





X 





mens, is attained gradually. The initial thecse 

 are provided with long, stout spines, which 

 may measure fully 7 mm. iu length. In some 

 cases, indeed, these thecse have lost their 

 original form, and have become entirely 

 modified to spines. The virgella is very 

 conspicuous, and may be as much as 11 mm. 

 in length ; it is partially surrounded by a 

 membrane similar to that noticed iu the case 

 of Climacog. caudatus. The long virgella 

 and spines of the initial thecoe give a 

 characteristic appearance to the proximal 

 end of this species. Another conspicuous 

 feature is the broad rod-like or tube-like 

 structure which runs through the polypary, 

 and is continued beyond the distal extremity. 

 This may be the virgula, but it seems more 

 likely that it is a structure which contains 

 the virgula. The septum appears to develop 

 between th. 5 1 and th. 5 3 . 



Iu bi- profile view the tliecas have a 



mg long virgella with its membrane and . . . . . „ . . 



thecal spines. Hartfell Spa, Lower Hartfell general inclination 01 about oO , the 1111- 



Shales. Coll. Elles. -, .. P . -, , -, -,-, 



pressed portion of the ventral wall passing 

 insensibly into the sacculate portion of the free edge (Fig. 159 a) ; in sub- 

 scalariform views, however, the denticle is always clearly defined and strongly 

 inucronate, and there is an appearance of constriction immediately beloAv the 



apertural edge (Fig. 159 c). This apertural margin 

 in specimens preserved in semi-relief is strongly 

 lobate. It is first notched interiorly, and then 

 rises to a broad lobe to become again depressed at 

 its extreme median outer limit, so that the mouth 

 of the theca resembles in form the lip of a jug. 

 After compression this apertural margin may appear 

 horizontal, inclined, or somewhat everted, and 

 rostrate or cuspate. These features are, however, 

 usually more clearly seen in examples of the varieties 

 of this species, especially in var. vulgatus (see Figs. 

 1G0 a— d). 



Affinities. — 0. calcaratus may as a rule be readily 

 distinguished from all other Orthograpti by the long virgella and large spines 

 belonging to the basal thecse. 



A 



a. Proximal end in full relief, showing origin of 



septal groove. Dobh's Linn, Lower Hail fell 

 Shales (zone of Dicranog. Clingani). Coll. 

 Lap worth. 



b. Proximal end of a compressed specimen, show- 



Fig. 159 c. — Orthograptus calcaratus, 

 Lapw. 









^ 





c. Distal theca?. Enlargement of part 

 of PL XXX, fig. 1 c. 



