PHACOPID^E. 13 



Beginning, then, with the family of the Phacopidse, as being on the whole the most 

 typical and highly organized, I may remark that there is less difference between the various 

 groups into which this natural family is divisible than between the various members of the 

 neighbouring families. So much is this the case, that palaeontologists have as yet 

 been generally unwilling to break up this group into genera, or to consider its divisions as 

 more than sub-genera of the great genus Phacops. Or, if they divided it, they have been 

 obliged to include a greater variety of forms in some of the divisions than in others. 

 Dalmania, or Dafmanites, is an example of this. It. was intended by its author. 

 Emmerich, to include only the broad expanded forms of the genus ; but M. Barrande, whose 

 authority has much weight, has widened its meaning so as to include all the forms which 

 have distinct lobes to the glabella ; thus including in Dalmanites both convex and flat 

 forms, in fact, four-fifths of all the species ; while the original term, P/tacops, includes 

 the rest. 



Prof. Goldfuss had previously taken the same view, but applied the term Acaste to the 

 larger group, leaving only the species with inflated lobeless glabella in Phacops. This 

 view is a consistent one, for Dr. Emmerich, in founding his genus, gave these last as the 

 type of it. But in the ' Neues Jahrbuch' for 1845, Dr. Emmerich objected to this plan, 

 preferring to unite all the more compact and convex forms in Phacops, whether with lobed 

 or lobeless glabella, separating only the more expanded forms, as Dalmania, a term 

 which, though in general use, had unfortunately been in previous employ for a group of 

 insects. Prof. Burmeister did not attempt to divide the group at all, and Prof. McCoy 

 included all under Phacops, while he recognized truly most of the subgeneric groups. 



It will be seen that there is a considerable diversity of opinion as to the value of the 

 subdivisions ; and this arises, I think, from the fact before noticed, that the various sub- 

 genera in this, the highest group, differ only by characters of proportion and degree of 

 development of the different parts, while the main features of the group remain constant. 



The great characters pointed out first by Quenstedt, viz., that the Phacopidae have eleven, 

 and only eleven, rings to the thorax, while other groups are variable in this respect, and 

 that all have the strongly facetted eyes, have in the eyes of naturalists overruled the 

 minor distinctions, and disposed them to undervalue the real differences of proportion 

 which exist. In the Decades of the Geological Survey, I have endeavoured to do justice 

 to all the above distinguished authors ; and, retaining the name Phacops for the whole, 

 pointed out the several natural sub-genera. I believe still we shall best consult the con- 

 venience of students by retaining the common name. 



No doubt, if we had the living animal, we should attach greater value to what in the 

 fossil appear subordinate characters. The degree of development of the eye, for instance, 

 should surely be a point of much importance in any group. The expanded form and 

 large size of one division, contrasted with the contracted dimensions and compact habit of 

 another, is certainly of consequence, and may well afford generic characters. 



And when we find the caudal margin in one form even and compact, while another has 



