Vol. 54.] FAUNA OF THE SKIDDAW SLATES. 505 



and continue throughout their whole length in more or less of 

 a straight line, though in some of the larger specimens there 

 is a tendency to curve upwards at the distal extremities. Some- 

 times the stipes attain an enormous length. A slab in the Keswick 

 Museum shows a large specimen, one stipe of which is complete 

 and measures 12 inches (30"46 cm.) in length; the total length 

 of this specimen must therefore have been originally at least 

 24 inches (61 cm.). Another specimen, now in the Woodwardian 

 Museum, must have been at least 21 inches (53 cm.) in length. 

 These are, so far as I know, the longest specimens on record. 

 The stipes are of considerable width even at their origin, and may 

 widen out to as much as ^ inch (3'17 mm.). This is rather in excess 

 of the measurement given by Hall, but he does not seem to have 

 had such large specimens under his notice. 



In none of my specimens have I noticed that the number of thecae 

 to the inch exceeds twenty-eight (eleven in 10 mm.) ; in fact, that 

 number is only exceptionally attained near the proximal end when 

 the thec£e are not typically developed, and even then the number is 

 more commonly twenty-six (rather more than ten in 10 mm.). 

 Nicholson gives thirty, thirty-two, thirty-four (twelve to fourteen 

 in 10 mm.) for some Skiddaw Slate forms, and he also figures 

 a specimen with concavely curved dorsal walls. None of my 

 specimens show this character at all, and I should be inclined 

 to refer Nicholson's figure to Didymograptus nitidus (Kail). The 

 width of the stipe varies, due no doubt in part to the variation in 

 the length of the thecse and their inclination ; in some young 

 specimens the maximum width is therefore never attained. As a 

 general rule the stipes are wide at their origin, and continue to 

 widen slightly towards their distal ends ; but immediately before the 

 actual extremity is reached there is a diminution in width, owing to 

 the partial development of the latest thecse. 



Structure of the proximal end. — The sicula attains a 

 length of J inch (3*17 mm.); this is rather long, though small in 

 proportion to the size of the rhabdosoma. It is long and narrow, 

 especially in the apical part : at its aperture it is J^ inch (about 

 •5 mm.) in width. The first theca (th. 1^) seems to originate very 

 near the apex of the sicula; it grows at first closely adpressed to the 

 sicula in a straight downward direction, but then curves away from 

 the sicula, leaving it free near its aperture for about one-third of its 

 length on the left side. The second theca (th. 1^) also at first grows 

 closely adpressed to the sicula ; it seems to arise from the connecting- 

 canal which is situated near the apex of the sicula. Theca 1^ does not 

 grow much farther down than theca V, consequently the proximal 

 part of the rhabdosoma in this species has an unusually symmetrical 

 appearance. The apertures of the earliest thecse are more nearly 

 parallel to the direction of the stipe than those subsequently 

 developed. In the obverse view the sicula is seen to be somewhat 

 obliquely placed, but this is not so noticeable in the reverse view, 

 when the right side and part of the aperture are concealed by the 

 growth of theca 1". 



Adult part. — The stipes are characteristically widip ; the average 



