GRAPTOLITES OF NEW YORK, PART 2 447 



The hydrothecae have features common to those of both Diplograptus 

 and Climacograptus. The)' are steeply inclined and have an oblique aper- 

 . tare, 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. 2;e] their walls appear to be ele- 

 gantly curved, and there is an appearance of distal expansion. In the 

 reverse aspect the walls are almost straight, and the thecae 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 compression. 



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 (visible 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 Clima- 

 cograptus, into a mucronate extension, oblique, and occasionally of remark- 

 able length. The middle division is short and approximately perpendicular, 

 as in Climacograptus. The final division is formed by the line of the aper- 

 tural 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 hydrotheca. 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 different from the 

 stout chitinous film representing the commoner diprionidian forms with 

 which it is usually associated. In the Girvan examples the test appears to 

 have been more or less punctate. 



Lapworth points out that Hall while not attempting to describe the 

 shape of the thecae, figures a specimen [see text fig. 410] in which the 

 thecae are inverted in position, i. e. the apertures opening towards the sicu- 

 lar end. This abnormal aspect is due to the recurving direction of the 

 oblique outer mucronate extensions of the thecae and well shown in the 

 specimen reproduced in text figure 411. The broadly oval to subcircular 

 apertures themselves are shown in oblique compression in figure 413, in 

 frontal view in figure 415. In the latter specimen the attenuated punctate 

 test contrasts with the solid weltlike marginal thickenings of the apertures 

 which appear as circular rings in the compressed specimen. The stoutest 

 ring surrounds the aperture of the sicula [see text fig. 415, 417]. This sicu- 



