HIBBERTIA ORBICULARIS. 207 



the former from the Coal-measures of Coseley, near Dudley, the latter from the 

 Carboniferous shales — so rich in Crustacea, Arachnida, &c. — of Eskdale on the 

 Scottish borders).^ 



Xone of the earlier fossil forms of Limuloii Crustaceans hitherto figured and 

 described, of which a series may be studied on pi. xxxi, figs. 1 — 8,' Monograph of 

 the Merostomata,' Palasont. Soc, part v, 1878, aid us in finding a suitable form for 

 comparison with the shield of Hihbertia, nor do the larval stages of the recent 

 Limiilus greatly assist in this direction (op. cit., pi. xxxiii). We are therefore 

 reduced to the conclusion that Hibbertia may be more conveniently placed along 

 with and near to those forms referred to the genus Cydus than with any other 

 group. Under these circumstances we preferred to regard it as characteristic of a 

 distinct genus ; and named it after Dr. Hibbert, with whose discoveries in the 

 Lower Carbonifei'ous strata at Burdiehouse it is most closely associated. 



From the paper on Cydus by H. Woodward, already referred to, we make 

 the following extract (pages 534 and 535), to show the conclusions arrived at by 

 the writer as to the affinities of the curious little forms included in the genus 

 Cydus. To the same family our specimen from Burdiehouse must now be added 

 under the name of Hibbertia. 



" The presence of antenna and biramous swimming-legs prove undoubtedly 

 that Cydus was a Crustacean. The large size of the former and the homogeneous 

 nature of the rest of the appendages (all biramous swimmiug-legs, with possibly 

 masticatory bases), taken in connection with the large, slightly bivalved carapace, 

 suggest that it is an Entomostracan and probably one of the Phtllopoda, with a 

 broad cephalic carapace like that possessed by Ajjus and by Daphnia ; with large 

 swimming second antennge like the latter, and possibly with a pair of stalked eyes. 

 Cydus, however, differs from the Cladocera in being flattened dorso-ventrally, and 

 from the lowest Crustacea in not apparently possessing any true jaw-parts — the 

 head, with the labrum and mouth, being bent further back than in the living 

 Entomostraca. These differences may either indicate very lowly characters or 

 very much specialised ones. Two views suggest themselves : 



" (1) That these animals were small, free-swimming Phyllopods, with expanded 

 cephalic shield, swimming second antennae, and biramous limbs, the bases of which 

 served as masticatory organs, no true jaws having yet been developed ; the back- 

 ward position of the mouth may have been brought about in order to allow as 

 many appendages as possible to serve as jaws, as is seen in Limulus. Or, possibly, 

 the beast could attach itself, like a living Daphnia, by a cement-gland on the 

 dorsal side of the head, in which case it might be an advantage to have the mouth 

 as near the freer end as possible and close to the swimming legs, which were, by 

 their movements, producing the foot-currents. 



1 'Trans. R)y. Sac. Edinburgh,' vol. xxs (18S.3), p. 227, pi. xwii, figs. 9— 9 </. 



