172 Mr. C. Lapworth on neio British Graptolites. 



spurs, visible only in the ventral aspect, while both the 

 lateral margins were wrinkled or undulated in the profile 

 view. Professor Hall can hardly be said to have attempted 

 the description of the shape of the hydrotheca?, in G. marcidus, 

 but rather to have contented himself with noting the form 

 and position of the marginal serratures. A glance at one of 

 his figures (fig. 2) shows some most remarkable features in his 

 fossil, not only difficult of reconciliation with his description, 

 but totally at variance with the view generally held of the 

 structure of the polypary in the genus Diplograptus. The 

 marginal denticles in the specimen figured, instead of being- 

 directed acutely upwards as in other forms of the diprionidian 

 Rhabdophora, project obliquely downwards, in the direction 

 of the proximal extremity of the polypary. If, therefore, 

 they stand to the individual hydrotheca? from which they are 

 derived in corresponding relations to those in the generality 

 of diprionidian forms, they must be inverted in position — their 

 apertures, instead of opening towards the distal end of the 

 polypary, must be turned proximally in the direction of the 

 initial extremity of the pol ypary. 



In 1868 we find Mr. W. Carruthers claiming the American 

 form as being identical with his previously described Diplo- 

 graptus tricornis (Geol. Mag. 1868, p. 131) ; and many grap- 

 tolithologists have subsequently supported this view. 



In 1872 Mr. John Hopkinson described and figured a form 

 of Diplograptus from the Moffat Shales of Wenlockhead, 

 under the name of Diplograptus Etheridgii (Geol. Mag. 1872, 

 p. 504), which appears to possess the essential characteristics 

 of Diplograptus tricornis, Carr., and to be very doubtfully 

 separable from that species. At first glance, however, it 

 would appear that the hydrothecse in the two forms are 

 strikingly distinct in shape. Instead of being rhomboidal, as 

 in D. tricornis, the thecas in D. Etheridgii are described as 

 having " the appearance of rounded knobs, their outer margins 

 forming a continuous curve, at first concave, and then, for 

 about half their length and round their apertures, convex." 



That these three forms, if not specifically identical, are, at 

 any rate, most intimately allied, I have long been convinced ; 

 but I have hitherto been baffled in my endeavours to show 

 that appearances so diverse as those noted above can possibly 

 be presented by one and the same diprionidian form. Fortu- 

 nately, however, I have very recently collected a large series 

 of specimens of a dwarf variety of D. tricornis from the 

 Lower Bala rocks of Girvan, preserved in partial relief. A 

 careful microscopic study of these specimens has enabled me 

 to gain a tolerably complete idea of the uncompressed form of 



