172 ■ F. U. Cowper Reed — On the Phacopidce. 



EXPLANATION OF PLATES. 

 Plate VII. 



Fig. 1 shows the orifjinal face A at the top of the stone. In front is the 

 cleavage plane E, which, however, is not clearly visible owing to the light 

 reflected from the somewhat irregular surfaces at the back of the ciystal. On 

 the extreme right is the cleavage plane H. 



Fig. 2 shows the cleavage plane F, which, on account of its favourable position 

 relative to the camera, appears as a brilliantly illuminated surface ; the 

 irregular original faces C and D also appear in this view. 



Plate VIII. 



Fig. 3 shows the triangular indentations on the irregular face D ; also portions of 



A and B. 

 Fig. 4 shows the crystal resting on the cleavage plane E with the faces B and D 



exposed to view. The sharp bounding edges are formed as follows : at the 



bottom by E, on the right by A, and on the left by G. 



VI. — The CiiAssiFicATioN of the Phacopid^. 



By F. R. Cowper Reed, M.A., F.G.S., of the Sedgwick Museum, Cambridge. 



f pHE family of Trilobites termed Phacopidee has been defined by 

 I Beecher ^ as follows: — "Glabella tumid, widest in front. 

 Free cheeks continuous, united anteriorly. Suture extending from 

 in front of the genal angles inward to the eyes, and thence forward 

 around the glabella. Eyes generally large, always with distinct 

 facets, schizochroal. Thorax of eleven segments with grooved 

 pleura. Pygidium usually large and of many segments ; limb 

 ribbed ; margin entire or dentate." 



Nomenclature in use. 



While the general limits and characteristics of the family as thus 

 given are universally recognised, much confusion and diversity 

 of opinion still exist as to the generic groups which must be 

 therein included. There is no precise agreement in the usage of 

 many of the common generic names ; and many subgenera have 

 been from time to time established without general acceptance. 

 Some palfeontologists (e.g. Salter) have been of the opinion that the 

 family only contained one gentis, Phacops, which might be split up 

 into several subgenera. Barrande, however, recognised the presence 

 of two genera, Phacops (in a more restricted sense than Salter) and 

 Dalmania or Dalmanites ; and Hall & Clarke in 1888- adopted this 

 view. Beecher in 1900 (op. cit.) mentioned six divisions or groups 

 within the limits of the family, and put them all as of equal generic 

 value. 



Other examples of differences in the classification of the family 

 need not here be given ; but in order to show the multitude of 

 generic or subgeneric groups which have been established, but of 

 which only a few have been generally adopted, the following list 



' Beecher, Amer. Journ. Sci., vol. iii (1897), p. 202 ; and in Zittel's " Textbook 

 of Paleontology " (English translation, 1900), p. 636. 

 2 Hall & Clarke : Palceont. New York, vol. A-ii, pp. xxvii-xxxii. 



