1918] 



The Ottawa Naturalist 

 REVISION OF SOME PHACOPID GENERA.* 



31 



By F. H. McLearn, Geological Survey. Ottawa. 



INTRODUCTION. 



In the preparation of a monograph on the fauna 

 of the Silurian Arisaig series, it has been found 

 necessary to revise the interpretation of several 

 Phacopid genera ; for the definitions now used are 

 not considered to be in accord with the evolution of 

 this trilobite family. Dalmanitina Reed, Phacopi- 

 della Reed and Phacops Emmrich are redefined. 

 The equivalent of Acasle Goldfuss and Acastc 

 Salter is pointed out. Porilockia McCoy is revived 

 and emended, with subgeneric rank, for the Silurian 

 ancestors of Devonian Phacops, s. str. The generic 

 name Clockeria Wedekind is found to be untenable. 

 Phacopina Clarke is not considered to be a 

 true Phacopinid and is placed in the subfamily 

 Dalmanitinae. 



In order that the proposed definitions of the above 

 genera may be established on a genetic basis, the 

 evolution of that portion of the family Phacopidae 

 concerned is treated first. 



EVOLUTION. 

 Both Hoernes (1880) and Reed (1905) recognize 

 in such Ordovician forms as Dalmania socialis 

 Barrande a generalized and primitive expression of 

 this family. The cephalon is characterized by well- 

 marked pentamerism, all the glabella furrows being 

 fully developed. The glabella is relatively high and 

 narrow, with the lateral borders subparallel, or at 

 most only slightly diverging. The genal angles are 

 rounded off or produced merely into short spines. 

 The pygidium has few segments and is rounded on 

 the posterior border. This generalized stock con- 

 tinues into the Silurian. It is represented there by 

 forms like Phacops ( A caste ) constricla Salter and 

 Cal^mene dorvningiae Murchiscn which show no 

 important modification of either the cephalon or 

 pygidium except rarely the slight mucronation of the 

 latter. Closely associated with the parent line are 

 a number of forms slightly modified in the direction 

 of the Phacopinas by obsolescence of the two anterior 

 pairs of glabella furrows. But the glabella is still 

 high, its borders are not conspicuously divergent and 

 the third pair of side lobes are not greatly reduced. 

 They cannot be regarded as Phacopinid. This 

 slight modification reoccurs in the Ordovician, Sil- 

 urian and Devonian. In the Ordovician this de- 

 parture from the primitive type is exhibited by 

 Dalmania phillipsi Barrande and D. solitaria 

 Barrande. The only important modification here is 



*By permission of the Director of tiie Geological 

 Survey, Canada. 



the obsolescence of the two anterior furrows. In the 

 Silurian a similar modification is shown in the 

 Arisaig Dalmania logani Hall, but since it exhibits 

 all gradations with a primitive form D. logani var. 

 conservalrix, n. var., it is thought to be independent 

 of the Ordovician forms and not derived out of 

 tnem. A Devonian departure from the primitive 

 stock of the same nature is to be found in such 

 species as Phacops hraziliensis Clarke and P. anceps 

 Clarke, which Clarke has incorporated into his 

 genus Phacopina. The change here is not much 

 more than in D. logani or the similar Ordovician 

 species and cannot be compared with the profound 

 modification of the Phacopina?. 



Several Ordovician species show a slight modifica- 

 tion of the normal Dalmanitina type by a broaden- 

 ing anteriorly of the glabella, but retaining the 

 primitive pentamerism. The third pair of glabella 

 lobes, although small, are not markedly reduced and 

 are tuberculose at the extremities as in the Phacopinae. 

 Such forms are Phacops ( A caste ) alifrons Salter 

 (1864, p. 33) and Phacops jamesii Portlock (Salter 

 1864, p. 32). Another modification of the gener- 

 alized line in the Ordovician is exhibited by the 

 species Phacops brongniarti Portlock (Salter 1864, 

 p. 34) which while retaining the primitive pentam- 

 erism of the glabella shows a considerable broaden- 

 ing anteriorly of the latter and a very considerable 

 reduction of the third pair of side lobes with tuber- 

 culation of their extremities. In the last character 

 this is a very near approach to the Phacopinae. This 

 subfamily, however, does not appear until the early 

 Silurian. 



In very late Ordovician or earliest Silurian time 

 the generalized line of the Phacopidae gave rise to 

 two quite far removed groups, both of which are 

 very distinct from the coeval primitive stock. On 

 the one hand, as Reed has noted (1905, pp. 176, 

 224), arose Dalmanites Barrande and related genera, 

 in which the frontal lobe becomes semi-detached 

 from the remainder of the glabella by the broadening 

 of the anterior pair of furrows, the genal angles 

 become produced into spines, the pygidium has 

 numerous segments, its axis becomes more slender 

 and it is nearly always mucronate or produced in a 

 spine. An intermediate form, Dalmanites n>eaveri 

 (Salter) appears in the early Silurian (Upper 

 Llandovery) in which the pygidium is not mucronate 

 and the genal angles are not produced into spines. 

 In the Devonian in particular this stock becomes 

 highly differentiated in spinescence, modification of 

 glabella, e\r. 



