A REVIEW OP THE TlilXOBITES. 285 



Phacops and calymene are frequently placed together as members 

 of the same family, but it cannot be too strongly insisted on, that 

 their characters are essentially distinct. Amongst those of a con- 

 stant value, we may cite — the number of the body-rings ; eleven in 

 the one form, thirteen in the other. Secondly, the conformation of 

 the glabella : large and expanding in phacops ; comparatively small 

 and contracted anteriorly in calymene. Thirdly, the direction of the 

 facial suture. Fourthly, the character of the eyes — and so on. Be- 

 sides which phi cops (or its kindred genus dalmannia) is most inti- 

 mately connected with ceraurus — the character of the glabella, the 

 facial suture, the number of the body rings, are the same in each — 

 a form with which calymene has certainly no relations. Hence the 

 two may be legitimately placed apart : each as the type-form of a 

 special group. 



In Part II. a brief analysis of the more important genera and 

 species belonging to these families will be given : shewing more fully 

 the connecting points between the various groups, and the data on 

 which the above arrangement is chiefly founded. In order, however, 

 to render the present Past complete within itself, a rapid enumera- 

 tion of the essential characteristics of each family i3 here appended : 



Illcenidcs — Buckler and pygidium large and smooth. Caudal axis 

 scarcely developed. Glabella feebly raised ; simple. Eyes far apart. 

 Body-rings 5-10. 



Brontidce — Buckler and pygidium large ; the latter with fan-like 

 furrows and very short axis. Glabella slightly raised, furrowed. Eyes 

 far apart. Body-rings 10. 



Asaphidce — Buckler and pygidium large ; the latter with well-de- 

 veloped axis, and usually with striated limb. Glabella simple (or 

 slightly furrowed). Eyes tolerably near together. Body-rings 8. 



Trinuclidce — Buckler large, horned ; generally with perforated 

 limb. Glabella oval, strongly pronounced. Pygidium of medium 

 size. Body-rings 5-6. 



Proetidce — Buckler and pygidium of good size ; the former border- 

 ed, the latter with well-developed axis. Glabella large, smooth (rarely 

 furrowed.) Body-rings 8-12. 



Ca lymenidiB — Buckler bordered, without horns. Glabella furrowed 

 g smooth, narrowing anteriorly. Facial suture terminating at the 

 angles of the buckler. Pygidium and its axis well-developed. Body- 

 rings 13. 



Harpesidec — Buckler large, crescented, with perforated limb. Gla- 

 bella narrowing anteriorly. Pygidium small. Body-rings 25-2G- 



