218 BRITISH GRAPTOLITES. 



may be regarded as tlie type — in which the form of the transverse section varied 

 from square (D. quadrimucronatus) to quadrangular with rounded angles (D. 

 calcaratus, etc.). This group was erected into a sub-genus by Lapworth in 1873, 

 under the title of Orthograptus. 



In a second group the form of the transverse section of the polypary was sub- 

 circular or elliptical. This group — which is typified by Diplog. tamariscus, Nich. — 

 constitutes the sub-genus Glyptograptus, Lapworth. 



In a third group the transverse section was distinctly concavo-convex, and the 

 appearances presented by compressed forms show a combination of those found 

 separate in Climacograptus and Orthograptus. This group, which includes such 

 forms as Diplog. foliaceus, Murch., may be distinguished as the sub-genus 

 Mesograptus (nov.) The extreme species of this group, in which the concavo- 

 convexity of the polypary is most pronounced, have been sometimes separated 

 off to form the sub-genus Amplexograptus (Diplog. per 'excavatus, Lapw.). 



Finally, there is a fourth group, in which the transverse section of the uncom- 

 pressed polypary was flattened from obverse to reverse. The majority of the 

 species of this group — which also exhibit a tendency to become more or less folii- 

 form in outline, and are typified by Diplog. folium, His. — are generally united under 

 the sub-generic title Petalograptus, Suess. The extreme forms of the group, in 

 which the polypary has a wedge-like outline, are typified by Diplog. cometa, Geinitz, 

 and constitute Hopkinson's sub-genus Cephalograptus. 



As regards the distinctions in the original shapes of the thccse in the genus 

 Diplograptus, we find at the one extreme, in the sub-genus Petalograptus, the 

 theca?, in the form of long cylindrical and but slightly flattened tubes of equal width 

 throughout, recalling those of the much older genus Phyllograptus, Hall. In the 

 typical forms of the sub-genus Ghjptograptus the theca is shorter and stouter, and 

 the middle third of its ventral wall impressed to form a distinct " excavation " as 

 in the genus Climacograptus; but the free edge is inclined instead of being 

 vertical, the sigmoid ventral curve is flowing rather than sharp, and the " excava- 

 tion " is wide instead of deep. In the sub-genus Mesograptus the earlier theca?, 

 as a rule, resemble those of Climacograptus and the later ones those of Ortho- 

 graptus', but in the obverse aspect of the forms ranged under Amplexograptus 

 the " excavation " in all the theca? is characteristically wide and deep, as in 

 Climacograptus. In most of the forms grouped under Orthograptus the general 

 form of the theca is that of a subquadrangular sac, expanding above and pressed 

 inwards below; but occasionally the ventro-dorsal narrowing of the theca is 

 carried so far throughout that the " excavation," as such, seems almost to 

 disappear, while the curvature of the ventral wall is at the same time so much 

 reduced that in extreme forms of this sub-genus, such as Orthog. quadri- 

 mucronatus, the theca? approximate in shape to flattened rectangular prisms. 



As respects the original shape of the edge of tin- thecal aperture and its 



