206 BRITISH PALEOZOIC PHYLLOCARIDA. 



from which the field of the buckler rises with a gentle swelling over and along 

 the crescentic area, except where the edge falls in for a little space just on the 

 centre of the front border. Where the buckler becomes slightly depressed and 

 the narrow groove along the margin is interrupted or perhaps injured there is an 

 obscure impression of what may be a jointed autennary organ in front. The 

 surface of the two lateral areas of the buckler have a finely granulated ornamen- 

 tation, which is most marked just within the peripheral rim of the carapace, and 

 is limited by two thin parallel granulated ridges, each starting from the incurved 

 posterior angles of the shield, the longest being 7 mm. and the shortest only 4 

 mm. in length; the intervening space of 5 mm. in breadth, marking the centre 

 of the carapace, is devoid of the granulated ornament seen on the other parts of 

 the shield, and has only a small spine-like impression 3 mm. long, and extending 

 for three quarters of the entire length of the shield as a curved irregular incised 

 line. There are some other markings, too obscure for interpretation ; also a 

 slight posterior projection on the central line, doubtfully representing part of a 

 lost series of caudal segments. 



The presence of the two corresponding angles, and the narrow posterior 

 openiug between them, suggest affinities with Ditliyrocaris (cf. I). Scouleri as 

 represented byM'Coy, PI. XXV, fig. 6a, with its two raised lateral ridges), but in 

 most of the members of this group the lateral portions of the shield are less 

 circular in outline, and in many the surface-ornamentation is composed of linear 

 or reticulate, and only occasionally of granulate markings. Compared with the 

 shield of Apus or Lepidurus, 1 the general outline is much alike, but the granulated 

 mesolateral ridges continued forward from each of the posterior angles in 

 Hibbertia are not present in Apus, whereas the median ridge so conspicuous in 

 both Apus and Ditliyrocaris is apparently quite absent in Hibbertia, or is not pre- 

 served in the fossil. Contrasted with the anterior (cephalic) buckler in Limulus, 

 the shield of Hibbertia is seen to be nearly circular, whilst that of Limulus is 

 semicircular ; the posterior angles of the shield of Hibbertia are contracted 

 together and directed somewhat inwards at their extremities, whilst in Limulus 

 they are wide apart and directed rather outwards. The mesolateral (ocular) 

 ridges are present both in Hibbertia and in Limulus ; but in Limulus they form a 

 smooth, curved, broad line, not a nearly straight ridge as in Hibbertia. 



The granulation on the lateral areas of the shield and on the ridges is also 

 strange to Apus, and more closely resembles that seen in some species of the Car- 

 boniferous genus Cyclus (cf. Cyclus Johnsoni, H. Woodw., 2 and C. testudo, Peach, 



1 See Dr. S. A. Packard's ' Monograph of the Phyllopod Crustacea of North America,' 1883, 

 pi. xvi, fig. 1, L. r/lacialis. 



2 See " Contributions to our Knowledge of the Genus Cyclus from the Carboniferous Formation 

 of various British Localities," by H. Woodward, ' Geo!. Mag.,' dec. 4, vol. i, 189i, pp. 530— 539, pi. xv. 



