DIPLOGRAPTUS. 259 



identified as Diplograpti, but when fragmentary or in sub-relief have often been 

 relegated to the Climacograpti. Forms belonging to the second group have been 

 classed as Glimacograpti or Diplograpti, according as the obverse or reverse aspect 

 of the polypary has been presented. 



GROUP I. 



Mesograpti in which the thecas were semicircular in section and the apertural 

 margins somewhat undulating ; excavations deep and conspicuous in proximal 

 region of polypary only. 



Diplograptus (Mesograptus) foliaceus (Murchison). Plate XXXI, figs. 8 a— f. 



1839. Graptolithus foliaceus, Murchison, Silurian System, p. 694, pi. xxvi, fig. 3. 



Polypary commonly short, about 3 cm. in length, with maximum breadth of 

 about 3 mm. ; margins parallel for greater part of length. Sicula small, 

 virgella conspicuous, lateral spines rarely preserved, unsymmetrically 

 disposed. Thecee fourteen to twelve in 10 mm., having an average length 

 of 2 mm., and overlapping one half to two thirds their extent ; apertural 

 margins slightly undulate, concave in profile view. 

 Description. — Compared with that of the majority of forms which have been 

 erroneously referred to it, the polypary of M. foliaceus is characteristically small, 



not exceeding 3 cm. in length. After widening 



Figs. 177 a and &.— Mesograptus folia- ° ° ° 



ceus (Mm-ch.j. rapidly in the proximal region it maintains a fairly 



i /, constant breadth for the remainder of its length. The 



v, L| r(V virgula is thread-like, and is occasionally prolonged 



£ ':', J j y t j for a short distance beyond the distal extremity. 



The sicula is rarely visible, but it appears to 



V,^ have had a length of at least 1 mm. ; it is, however, 



a b usually concealed in the reverse aspect of the 



a. Proximal end, obverse aspect, show- polypary. Details regarding the proximal end are 



liSSf^ obscure, but the presence of a virgella can usually 



b. Proximal end ^reverse aspect, hut fa detected. Oil til. I 1 a Spine is given off close 



showing sicula. Ibid. x ° 



below the aperture ; while on th. I 2 the spine is 

 further removed from the aperture, and seems to be given off where change in the 

 direction of growth occurs. 



In the proximal region of the polypary the thecas are closely set, and as shown 

 in bi-profile view nearly one half of the visible ventral wall of the theca is 

 impressed. In the distal region the amount of impression dies out, so that the 



