OF THE SPECIES. 95 



13. Ph. nmcronatus : Oculis mediis, lobo capitis sccundo ct tcrtio aiquantibus, caudic iliaclii 10-1 1 

 articulata, costis hiteralibus 8-9, scuti apice mucronato. Long. 8-4". 



Ecf. — Asaph. )Hncro)iatu-<t, Brongn. C'r.fo.ss. 24, PL III, Fig. 9. D.xlm. Pulcrad. 42. 1. 05, 

 Tab. II, Fig. 3, a-h. Schloth. Nachtr. \\, 37. 24. Milne Euw. Crust, iii, 308. 4. 



Eiüommtr. caudattts, Wahlenb. N. A. Upsal. viii, 28. 4, Tab. II, Fig. 3 ; Journ. d. Phys. 

 V, 91, p. 34, Fig. 4. 



Phacops mucronatus, Emmr. Dissert. 24. 11. 



Asaph, hngkatidatus, MuRCH. <S'/7. Si/.^t. 656, PI. XIV, Figs. 11-14. Milne Edw. Cr. 

 iii, 308. 3. 



Loc. — The Silurian limestones of England (Dudley, Wenlock), of Sweden (Ostgothland at 

 Borensliult, Schonen at Rostanga), in the clayslate of Mosseberg; likewise in the grauwackc 

 rocks of the Eifel at Daun (Sack's collection) associated with Tlomahnotus armatns. 



This species resembles the tenth in point of structure of the head, and the eleventh 

 with regard to the caudal shield, and thus forms an intermediate link between them. 

 According to Murchison's figure, its cephalic shield is pointed in the centre of the anterior 

 margin, and has a glabella which is not so broad anteriorly with the first lobe, decidedly 

 smaller than iu the preceding species, but which in front penetrates with a slight 

 point into the marginal point of the shield. The eye, rather smaller than in Ph. 

 caitdatiis, but much smaller than in Ph. Hnifsmanni, neither projects beyond the second, 

 nor at the posterior part beyond the third lobe of the head ; the long spines of the 

 posterior angles are more acutely prominent, and reach beyond the centre of the body. 

 The lateral lobes of the body joints are pointed. The caudal axis is longer and more 

 slender than in Ph. mudatiis, and consists of from ten to fourteen rings, the last four being 

 more or less distinctly separated ; at the sides there are only eight ribs, of which the first 

 six behind the foremost marginal rib exhibit a deep diagonal transverse furrow, which 

 is particularly distinct in specimens without the shell ; the margin is much narrower than 

 in Ph. caudatiis, and not broader towards the posterior part, owing to which the long spine 

 of the extremity usually issues more suddenly from the margin of the shield ; the spine 

 extends itself in the shape of a convex protuberance as far as towards the end of the axis, 

 and becomes as long as all its joints. 



Remarks. — 1. Though I only know this species from the descriptions of the authors eniuiierated, 

 aud tlie cephalic shield alluded to in Sack's collection, I am, however, convinced of its distinctness. 

 Formerl}' I attributed to it fourteen lateral ribs on the caudal shield ; but, owing to the diagonal 

 transverse fuiTow, I am now aware tliat I counted the anterior five twice over ; there are, in fact, only 

 eight lateral ribs. 



2. A number of species occur in Green's Monograph aud its appendices, which do not seem 

 to be different from Ph. caudatus or Ph. mucronatiis ; I content myself here by enumerating them; 

 they are founded for the greater part upon caudal shields. Ä.i(ipli. crypt urus. Green (Transact, of tlie 

 Geolog. Soc. of Pennsylvania, i, 37, Plate \1), Harlan {Med. et Pliys. Res. .303), !Milne Edwm-ds (iii, 

 313), a caudal shield with twelve rings of the axis, and ten ribs ; judging from the form, appears to 

 belong to Homalonotus. Asaph, limuliiriis [Mon. 48), Milne Edwards's species (iii, 307,) appears to be 

 identical with Ph. mucronatvs. Asaph, pleuropiya [Mon. 55) belongs either to the latter mentioned 

 species, or perhaps to Ph. Hausmanni. Asaph, mio-'irus {Mon. 56) is likewise a Phacops with a pointed 

 caudal shield, the specific characters of which cannot be more accurately ascertained, and which probably 

 belongs to one of the three species here described. 



Rather more diflcrent from each other are some caudal shields with two eud-poiuts, which probably 

 also belong to this genus ; I saw plaster casts of them, or at least of the second species, at Berlin. 



