234 BRITISH GKRAPTOLITES. 



spines on the two proximal thecae. Thecae thirteen to ten in 10 mm., 

 having a length of 2 mm., and widening somewhat abruptly at the edge of 

 the aperture, overlapping one half to two thirds of the total length. 

 Apertural margin slightly undulate, everted in bi-profile view. 

 Description. — The polypary varies in size with age, but always attains con- 

 siderable width. It is 1 nun. wide at origin, and, increasing at first rapidly and 

 then more gradually, attains the maximum breadth at the distance of about 1'5 cm. 

 from the proximal end; it may, however, diminish again distally, giving the 

 , , „ characteristic fusiform appearance. 



r ios. 154 a and b. Orthogra/ptus trun- l L 



catns, Lapu. The sicula is long and slender, measuring 2*5 mm. 



in length, but this is only determinable in young 



specimens, since its apex is entirely concealed in the 



v adult. Th. I 1 originates a little distance above the 



aperture of the sicula, but quickly embraces it; it 

 ;V grows down below the level of the aperture, but then 



bends abruptly round to grow upward and outward, 

 giving off a spine where charge in direction of 

 growth takes place (see Figs. 155 a — <■) ; th. I 2 , 



"' ^yMyLna^Gi^an^HStfeii whicD is also furmsned witn a mesial spine, develops 

 shales. Coil Mrs. Gray. from th. I 1 , and grows across the sicula iii such a 



b. Distal tneca; ol same specimen, snow- ° 



ing characteristic bi-profile view of manner as to conceal it completely from view ill the 



thecae, and growth lines. l 



reverse aspect of the polypary ; th. 2 1 appears to 

 cross in front of the sicula, which is, therefore, eventually almost concealed in the 

 obverse aspect also. The sicula itself is not entirely concealed till three thecae 

 have developed on each side. No septum makes its appearance till at least four 

 thecae have developed on each side, and it is doubtful if it is ever complete, for it 

 is, as a rule, invisible in the reverse aspect of the polypary, and the thecae 

 appear to develop alternately throughout, 



In the Scottish specimens the virgella commonly has a length of about 1 mm., 

 but in specimens from Shropshire this dimension is considerably exceeded. 



The thecae have an average length of 2 to 3 mm., widening throughout, but 

 somewhat abruptly immediately under the apertural margin. In the common 

 bi-profile view they appear to have a general inclination of 35° to 10°, and the 

 apertural margins are typically everted, and transgress slightly upon the wall of 

 the theea immediately above, while the overlapping of the cells is particularly 

 clear and— owing, perhaps, to the tenuity of the periderm— growth lines are 

 usually well marked. The distal prolongation of the virgula is characteristically 

 short and inconspicuous. 



Affinities. — Orthograptus truncatus is, apparently, allied to 0. caharatus and its 

 varieties, with which it is commonly associated; it should, however, be readily 

 separated on account of the more distinct eversion of the apertural margins in 



