PARABOLINELLA. 63 



those of the later segments progressively more and more directly backwards ; all 

 the pleura? deeply grooved by oblique furrows which are continued into the spines. 



Tail somewhat triangular, without any raised margin. Axis broad, conical, 

 reaching to the posterior margin, formed of four segments (including the terminal 

 portion), of which all but the last bear a median tubercle. Lateral lobes flat, 

 composed, apparently, of three pleurae similar to the posterior thoracic pleurae, 

 and each produced into a long backwardly-directed spine, the first pleura nearly 

 at right angles to the axis, the others oblique. Besides the three spines on each 

 side there is a pair of spines immediately behind the axis. 



Dimensions. — Very variable ; length from 8 to 30 mm., commonly about 25 mm. 



The form with which Parabolina spinulosa is most likely to be confounded is 

 Olenus cataractes. In perfect specimens the differences are sufficiently obvious; 

 but, as has already been remarked in the account of 0. cataractes, compressed 

 specimens of that species often appear to bear long thoracic and pygidial spines. 

 They may, however, be distinguished by the more backward position of the eyes 

 and the absence of axial tubercles ; and a close examination will usually show that 

 the appearance of long spines is due to wrinklings on the surface of the slate in 

 which the specimen lies. 



Some of the British specimens of Parabolina, are very small, and in some the 

 spines of the tail and thorax seem to be very short, so that at first sight they 

 appear to differ from P. spinulosa. I have, however, been unable to recognise any 

 of the other species described by Moberg 1 ; and the shortness of the spines seems 

 to be due in some cases to the youth of the individual and in others to the fact that 

 the extremities are imperfectly preserved. The specimen from Mr. G. J. 

 Williams's collection shown in Plate VI, fig. 11, is one of the most distinct of these 

 forms ; but other specimens, which Mr. Williams has kindly lent me, and which came 

 from the same locality, seem to indicate that there are stages intermediate between 

 this and the normal Parabolina spinulosa. For the present, therefore, I look upon 

 these small and short-spined forms as stages in the development of that species. 



Horizon and Localities. — Upper Lingula Flags : Rhiwfelyn, Craig-y-Dinas, 

 Mawddach Valley; Nant-y-derbyniad ; Nant Cistfaen, Llyn Tryweryn ; Bryn 

 Cyfergyd, Cwm Cynfal ; Carreg Wen, Borth ; Penmorfa ; Penmaenpool ; Grwern- 

 y-barcud, Dolgelly. 



Genus PARABOLINELLA, Brogger. 



The genus Parabolinella resembles Parabolina in some respects, but possesses 

 an entire instead of a spinose tail. There are also other important differences, 

 and the genus appears to be connected (through Olenus longispinus) with Olenus 

 proper rather than with Parabolina. 



1 Geol. Foren. Stockholm Fork, vol. xx, pp. 259—277. 



