122 F. R. Coicper Reed — Evolution of C/ieirunts. 



(2) the ver}' weak development of the two anterior side-furrows. 

 Though in Cyrto. ? aries the basal lobes are not completely circum- 

 scribed, yet the whole head-shield resembles in many important 

 features Cyrto.? psendohemicranium, and Schmidt (he. cit. p. 123) 

 holds that they are closely allied. Eichwald '' considered Cyrto. ? aries 

 to be a species of Sphatrocoryphe ; Hoffmann 2 held it to belong to 

 SphcerexocJius ; and Nieszkowski 3 and Steinhardt 4 also place Cyrto.? 

 pseud oh emicr cmium in Spheerexoehus. The pygidium ascribed to Cijrto.? 

 pseudohemicranium also shows quite peculiar characters. Hemispharo- 

 coryphe would be a suitable name for these forms. They have some 

 points of similarity to Ch. globosus (Barr.) from Dd -i and Dd o of 

 Bohemia, a point noticed by Schmidt (loc. eit. p. 128). This 

 Bohemian species, while possessing eleven segments, the number 

 typical of Schmidt's Section I, and constricted pleurae with hamate 

 processes to the articulating bands, has merely a longitudinal row 

 of puncta on the inner portion of each pleura instead of a furrow. 

 It has also an apparently complete communication of the marginal 

 with the axal furrow, and the basal lobe of the glabella is cut off 

 by the union of the third side-furrow with the neck-furrow. Its 

 pygidium is, however, very different from those of Cyrto. ? pseudo- 

 hemicranium and Cyrto.'? aries, being furnished with four pairs of 

 pleura? with free terminations, allying it to Ch. tnruescens and other 

 Bohemian species. I am therefore inclined to think that Ch. ylobosus 

 belongs to a small side branch, and is not in the direct line of 

 ancestry of later forms. 



Cyrto.? pseudohemicranium and Cyrto.? aries form a connecting 

 link between Cyrtometopus and Sphcerocoryphe, as before mentioned. 

 The latter subgenus has, in common with Cyrto.? pseudohemicranium, 

 the inflated anterior portion of the glabella, the separated basal lobes, 

 and the faint first and second side-furrows ; and, so far as is known, 

 the pleurae are similar in both cases. But the reduction in the 

 number of the thoracic segments to eight or nine (according to 

 Angelin), the much greater inflation and protuberance of the 

 glabella, and the lateral teeth to the head-shield, are important 

 differences. The pygidium has three pairs of pleurae according to 

 Kutorga, and has the appearance of considerable deviation from the 

 ordinary types in the genus Cheirurus. Spharocoryphe, in fact, 

 appears to be at the end of a highly specialized small side-branch, 

 which died off in Silurian times. 



The Staurocephalus? unicus, described first by Wyville Thomson ' 

 from the Bala beds of Ayrshire, is really a Sphcerucoryplte, and 

 seems to be the only British representative of the subgenus. On 

 the Continent the subgenus is confined to Ordovician strata. 



1 Eichwald. Lethtea rossica, 1860, p. 1408, T. lii, fig. 31. 



2 Hoffmann. Sammtl. Trilob. Russl. in Tern. d. mineralog. Gesellsch. 185S, 

 p. 30, T. i, fig. 5. 



3 Nieszkowski. Zusatze z. mono^r. d. Trilob. d. Ostseeprov. ira Archiv. fur 

 Natuxk., Liv. Ert imd Kurt, 1858, Ser. I, Bd. ii, p. 376, T. ii, figs. 7 and 8. 



4 Steinhardt. Die in preuss. Geschr. gef. Trilob. in Beitr. z. Naturk. Preussens. 

 D. phys. bkononi. Gesellsch. z. Konigsb. 1874, p. 60, T. iv, fig. 17. 



5 Quart. Journ. Geol. Soc, vol. xiii (1857), p. 209, pi. vi, figs. 13 and 14. 



