F, R. Cotcper Reed — On the Cheiruridce. 213 



of the free cheeks, must be taken as of considerable importance. The 

 marginal spines on the head-shield of Staurocephalus Murchisoni may 

 perhaps be regarded as merely the multiplication of the one or two 

 pairs present in Splicerocoryphe, but a marginal fringe of spines is 

 of but little morphological value, for we find it in the most widely 

 separated forms, such as Areia and Acidaspis, as well as in closely 

 allied species. The cheeks of Staurocephalus are not pitted like 

 most of the Cheiruridge, but are tuberculated, and in this respect, as 

 well as in the stalked eyes, resemble some of the Encriuuridas. But 

 surface ornaments may have too much importance attached to them, 

 and I doubt if they are often of more -than specific value. The 

 thoracic segments show a considerable resemblance to those of some 

 genera of the Encrinuridae in their shape, their rounded and ridged 

 surface, and the fulcrum, beyond which they are sharply bent down. 

 These features are suggestive, and seem to be of considerable value 

 in this case in indicating the relationship of this strange genus. 

 A furrow exists on the anterior edge of the pleurae of St. Murchisoni, 

 as Salter described, and a similar one occurs in the same position in 

 Mncrinuriis. This furrow may perhaps not be homologous with the 

 ordinary diagonal or longitudinal pleural furrow of other genera, but it 

 is a common feature in Micrinurus and Staurocephalus which is worthy 

 of notice. In Sphcerocoryphe the thoracic pleurae are not ridged, 

 and the furrow which is present on them runs along the central 

 line. However, we are not precluded from supposing the possibility 

 of its obliteration when we recall the case of Sphcerexochus and 

 I'seudosphcsrexochus. In the pygidium of Staurocephalus we see 

 again the impracticability of deciding affinities by this member. 

 The pygidium of one species (St. globiceps) shows a close re- 

 semblance to that of Sphcerocoryphe unicus, and in each form the 

 first pair of pleuree is much enlarged at the expense of the others. 

 But the less specialized pygidium of St. Murchisoni is completely 

 different, and while retaining the Cheirurid character of a small 

 number of segments, yet in its general shape, and the form and 

 coui'se of the pleurae, it strikingly reminds us of some species of 

 Amphion, and, more remotely, of some species of Cybele and 

 Encrinnrus. Barrande has remarked that its pygidium shows 

 a particular analogy to that of Ch. tumescens. From the foregoing 

 consideration of some points in the anatomy of Staurocephalus I am 

 led to conclude that its affinities are rather with the Encrinuridae 

 than with the Cheiruridee, and that its resemblance to Sphcerocoryphe 

 is more superficial than real, and is probably an instance of iso- 

 morphism. The pygidium in any genus is too variable a feature 

 to be of much use in determining true relationships, but in the 

 simplest and least modified form of this member, as represented 

 in St. Murchisoni, it is deserving of notice that there are indica- 

 tions of an alliance with the Encrinuridse. It may here be remarked 

 that in the latter family the large number of segments on the 

 axis of the pygidium probably does not represent so lai'ge a number 

 of coalesced pj'gidial segments, but is due to the secondary sub- 

 division of the original segments. Finally, we must cou^ie to the 



