8 BRITISH FOSSILS. 



the lobes being strong and deep, and for possessing eleven joints in the thorax, a number 

 apparently never deviated from. Besides these, in a large part of the genus the convex 

 form of head and tail, the curvature of the segments of the thorax, and the sharpening of 

 their anterior margins, eminently fit the animal for rolling up into a compact ball ; for this 

 purpose it is as completely constructed as Calymene, while it has the advantage over the 

 latter genus in its large organs of sight. It may be considered as the very type of a Tri- 

 lobite, possessing all the characteristics of the tribe, with a compact and elegant form. And 

 while the whole group is so bound together by common characters as to render it difficult 

 to separate it into distinct genera, there is a great variation of form among the numerous 

 species. Quenstedt, and after him Emmerich, united them all, and Professor Burmeister 

 has not ventured to dismember the genus. Subgenera, however, have been frequently 

 pointed out, and the groups so formed are very natural. We have 



1st Convex species, with the pleurae rounded at the end, and sharpened for rolling ; the 

 glabella inflated, and with but one lobe at the base ; the facial suture beneath the margin 

 of the head in front ; the head angles rounded, the tail of few joints. Portlockia, M c Coy. 



2nd. Moderately convex, with the pleurae rounded ; the glabella not inflated, but with 

 (3) distinct lateral lobes ; facial suture marginal in front ; head angles rounded, or 

 variously produced, tail few (less than 11) jointed, rounded or pointed. PJiacops, 

 Emmerich. 



3rd. Characters as given above. Dalmannia, Emmerich. 



4th. Flattened ; pleurae pointed, or even mucronate ; glabella and facial suture as in 

 Dalmannia ; head angles long-spinous ; tail many-jointed, the margin produced into spines. 

 Cryphceus, Green; Pleuracanthus, Milne Ed w. 



Odontochile, Corda, is synonymous with Dalmannia, and his Asteropyge and Metacan- 

 thus apparently with Cryphaus. 



The genus is found in all Silurian strata, and existed till the commencement of the 

 Carboniferous system. The distribution of the various subgenera in Britain is as follows : 



Portlockia is found in all the above strata. P. Sloltesii, Milne Edw., and P. sublavis, 

 M c Coy, Silurian. P. Latreillii, Steininger, P. yranulata and P. Itevis, Munster, Devonian 

 and lower carboniferous rocks, Devonshire and Cornwall. 



Phacops is Silurian. P. Odini, Eichwald, P. Brongniartii, P. Dalmannii, P. Jamesii, 

 Portlock, Lower Silurian. P. Downingia, Murchison, Upper and Lower Silurian. 



Dalmannia, Upper and Lower Silurian. 



Cryphtsus, Green [ C. callitelus, &c., so named before the definition of Milne Edwards' 

 genus, Pleuracanthus], Devonian. A specimen of C. arachnoides, Goldfuss, found at 

 Torquay, Devonshire, is in the collection of Mr. D. Sharpe. 



J. W. SALTER. 

 June, 1849. 



