212 F. R. Coicper Reed — On the Cheiruridce. 



to be a frequent tendency in the CheiruridEe for the posterior pairs 

 of pleurae to be reduced, and we find it remarkably exhibited 

 in Nieszhoivskia. In regard to Deiphon, though the morpliological 

 value of its various characteristics may be a matter of dispute, yet 

 its high degree of specialization must be generally acknowledged, 

 and we would place it at the end of this branch of the family. 

 Beecher^ has also briefly stated that he is of this opinion, and he 

 associates with it the Australian genus Onycliopyge. 



Turning now to the aberrant and imperfectly known form Youngia, 

 distinguished by Lindstrom''^ as a separate genus, we find there are 

 only three species established, and of these only the head-shields 

 have been discovered. One of them [Y. trispinosus) is found in the 

 Penkill mudstones of the Girvan district. The characters of its 

 head-shield link it on the one hand with PseudospJicerexoclius, and on 

 the other with Sphcerexochus. The spiniform fixed cheeks recall 

 Deiplion, but their development is not so extreme ; the neck spine is 

 a marked feature, but does not seem to be of phylogenetic importance. 

 In some species of Acidaspis and Liclias a spine is developed in the 

 same place, while other closely allied species are destitute of this 

 ornament. Bernard^ has compared this organ, which is often a mere 

 tubercle, with the dorsal organ of Apus, and suggests that it was 

 poisonous. 



Lastly, the genus Staurocephalus demands our attention. This genus, 

 though frequently stated to be allied to SphtBrocoryphe on the strength 

 of its abnormal glabella, possesses in reality many important dis- 

 tinctive features. The hypertrophy of a portion of the glabella has 

 in my opinion more of a physiological than of a morphological value. 

 In the style and extent of the inflation of the glabella of these 

 two genera a considerable difference is found to exist on careful 

 examination. In Staurocephalus it is only the frontal lobe which is 

 inflated and projects so conspicuously over the front border of the 

 head-shield ; the first side-furrows are very strong, and unite across 

 the glabella in a continuous groove, thus sharply marking off the 

 globular frontal lobe. The hinder portion of the glabella is parallel- 

 sided, and has two pairs of furrows marking off the lobes distinctly, 

 but the basal lobe is not circumscribed. In Sphm'ocoryphe excessive 

 development is shown by the glabella as a whole, with the exception 

 of the basal lobes, which are in the form of nodules. The facial 

 suture in Staurocephalus cuts the outer border of the head-shield 

 posteriorly, so that the free cheeks are of a fair size, but in 

 Sphcsrocoi-yphe the free cheeks are relatively very much smaller 

 and of a different shape, owing to the forward course of the posterior 

 branch of its facial suture. In fact, the two branches of the facial 

 suture in Sphcerocoryphe meet at a very acute angle, while in 

 Staurocephalus they meet at almost a right angle. This feature, 

 from what we now know of the ontogenetic and phylogenetic history 



1 Amer. Journ. Sci., vol. iii (1897), p. 201. 



2 Ofver. Kongl. Yet. Akad. Forhandl., 1885, No. 6, pp. 49-51, T. xiii, figs. 11, 12. 



3 Bernard, Q.J.G.S., vol. l (1894), pp. 422-4. 



