12 PHACOPIDiE. 



pleurae as at (b*). The furrows (b b) correspond to the pleural grooves of the thoracic joints, 

 and the finer intermediate lines mask the sutures between the several segments of which 

 the tail is compounded. The anterior groove (A) is generally the strongest. The margin 

 (e) is mostly smooth and even, and often concave. At (d) the facet is seen on the front 

 edge ; (/) is the mucro. either short or prolonged into a spine, and often absent altogether. 



These are not all the points of structure visible on the crust of the Trilobite. But it 

 is desirable not to multiply terms. I prefer the ordinary appellations head and tail to the 

 more correct designations, carapace and post-abdomen. The axis is a convenient and well 

 understood term for the middle portion, and is better than tergum ; and the terms side- 

 lobes, limb, or lateral portions of the tail ; free and fixed cheeks, &c. to the head, are suffi- 

 ciently intelligible and well known to render it unnecessary to give the more technical 

 designations " epimera," 8fc, even if we were quite sure of the correctness of all of these. 



It is indeed quite possible that the free cheeks, with the connecting portion in front, 

 constitute the first or ocular ring, and the rostral shield would form the sternal portion of 

 the same ring. But there is not absolute certainty of this, and some naturalists disbelieve 

 it. The rostral shield may be the only representative of a small anterior ring, and in that case 

 we should be only inventing prematurely a new term, in deciding to call the first segment, 

 as McCoy has done, the ocular ring. Nor would it be so convenient in description. 



Again, the glabella certainly covers the region of the stomach, and the glabella-furrows 

 doubtless mark the attachment of the muscles of the several segments proper to the head, and 

 covered by the expanded carapace. There are always three (and in Ogygia and some other 

 genera, four) of these lateral furrows ; and if they indicate the number of segments combined 

 in the carapace, instead of the number of thorax-segments covered by it, we should still be 

 at a loss whether to call the neck-segment, which is always combined with the carapace, 

 the fifth or sixth segment in the general plan. But by adhering to the terms " free cheeks," 

 "facial" and "rostral sutures," "rostral shield/' "glabella-furrows," "neck-segment," " tail," 

 &c, while there is no false assumption in the terms, so neither are they too far removed 

 from a scientific and technical nomenclature to be useful. 



It may before long be practicable to adopt a more complete terminology. But in 

 these descriptions I adhere to Dalman's formula, modernized a little to adapt it to our 

 more advanced knowledge of the group. 



Family— PHACOPIDtE. Emmerich, Corda, Salter, &>c. 



Eyes largely facetted, the cornea convex over each facet, forming a granulated, not a 

 smooth eye. Facial suture ending posteriorly on the outer margin of the cheek. Thorax 

 with eleven rings. Includes at present only one genus, Phacops. {Sab-genera. — Tiu- 

 mkrocephalus, Phacops, Acaste, Chasmops, Odontochile or Dalmania, Cryphceus.) 



