Mr. C. Laj) worth on new British Graptolites. 173 



the polypary, and of the appearances which are naturally 

 presented by it when compressed in different directions. 



In the ventral aspect the appearance of the polypary is (as 

 shown by Carruthers and Hall) precisely similar to that in 

 Climacograptus, except that the apertures, instead of being 

 semicircular in form, are subrectangular. When the fossil is 

 preserved in partial relief these apertures seem to occupy 

 about two thirds of the ventral aspect ; but a study of nume- 

 rous examples proves that only the lower half of this vacant 

 space is formed by the true aperture, the upper half being- 

 occupied by the fractured matrix filling a distinct " excava- 

 tion" (see PI. V. fig. 27 c). In profile views (figs. 27 b, 27 c) 

 the characteristic form of the hydrotheca is well shown, and 

 we see at a glance the cause of the appearance presented by 

 such forms as Hopkinson's D. Etheridgii. The hydrotheca? 

 have features common to those of both Diplogro/ptus and Gli- 

 macograptus. They are steeply inclined and have an oblique 

 aperture, as in the former, while their apertural margin opens 

 wholly within the ventral margin of the polypary, in a distinct 

 excavation, as in the latter genus. In the obverse (?) aspect 

 (fig. 27 e) their walls appear to be elegantly curved, and there 

 is an appearance of distal expansion. In the reverse aspect 

 the walls are almost straight, and the thecas are of equal 

 width throughout. These diverse appearances are, in all 

 probability, a result of the original form of the polypary, 

 which was somewhat concavo-convex previously to com- 

 pression. 



The outer portion of each hydrotheca forming the wrinkled- 

 looking ventral margin of the polypary is composed of three 

 divisions. The lowest division is a distinct excavation (visi- 

 ble directly only in subscalariform views) which overhangs the 

 aperture of the theca immediately below. The outer sinus of 

 this " excavation " is prolonged, as in many species of Climaco- 

 graptuSj into a mucronate extension, oblique, and occasionally 

 of remarkable length. The middle division is short and 

 approximately perpendicular, as in Climacograptus. The 

 final division is formed by the line of the apertural margin. 

 The latter is very oblique with respect to the axis of the 

 polypary, but, as in the majority of other Diprionida, is 

 almost at right angles to the normal direction of the hy- 

 drotheca. It lies wholly within the ventral boundary of the 

 polypary, and is visible in very rare cases. As pointed out 

 by Mr. Hopkinson, the test in these forms is of remarkable 

 tenuity. It is generally preserved as a mere stain, very dif- 

 ferent from the stout chitinous film representing the commoner 

 diprionidian forms with which it is usually associated. In 



