I9I81 



The Ottawa Naturalist 



35 



As already noted, the Silurian group of Phacops 

 stokesii (Milne-Edwards) is ancestral to the group 

 of P. latifrons Bronn and differs from the latter by 

 having the glabella more depressed anteriorly, so as 

 not to overhang the anterior margin, the size is 

 smaller, and the surface less tuberculose. They are 

 here placed under the subgenus Porlloclfia McCoy 

 emend., with P. stol^esii as the genotype. Among 

 the species referred to it are: 



Cal^mene stol^esii Milne-Edwards. 



Trilobiies elegans Sars and Boeck. 



Phacops orestes Billings. 



Phacops (Portlocl(ia ) marlflandensis, n. sp. 



As described and applied by McCoy in 1 846, 

 Porllocffia McCoy had a rather wide interpretation 

 and included both Silurian and Devonian forms with 

 the typical Phacopinid structure. It fell within 

 Phacops Goldfuss and was practically equivalent to 

 subgenus Phacops Salter, embracing both the P. 

 stol^esii and P. laiifrons groups. McCoy (1846; 50) 

 states that "this genus includes Cal])mcne tuher- 

 culata and C. macrolhahna of the Silurian system 

 [=^Phacops latifrons Bronn and P. sto}(esu Milne- 

 Edwards according to synonomy of Salter (1864; 

 18, 21)]; Phacops tiiberculata of Captain Port- 

 Icck's Report; Calvmene nupera Hall; Calymene 

 bufo Green, etc." In 1846 he (1846, p. 51) de- 

 scribed and placed in this genus Porilocl(ia sublaevis 

 McCoy. Salter considers this a synonym of P. 

 slokesii (1864, pp. 21, 22). Later he (1855; 162) 

 says: "The Cal])mene bufo of Green, C. niacro- 

 ihahr.a of Murchiscn [=P. s^o^esn], etc., being the 

 types of the genus". Since P. latifrons is the type 

 of Phacopi Emmrich and Calymene bufo Green is 

 a similar Devonian form, Phacops should be re- 

 stricted to that group, while the group of P. stol^esii 

 is now separated under the subgenus Portlocl^la. 



Genotype: Cal^mene stol(esn Milne-Edwards. 



Phacops ( Portlocl(:a ) marklandeiuis, n. sp. 



( Marlfland, Nova Scotia in Scandinavian 



mythology). 



Compared with Phacops ( Portlocf^ia ) elegans 

 (Sars and Boeck) this species has a still more re- 

 duced basal (third side pair) lobe and smaller 

 tubercules at its extremities. It differs in a similar 

 manner from P. ( Portlocl(ia ) stol(esii Milne-Ed- 

 wards and in addition in the presence of well de- 

 veloped, although not highly elevated, tubercules at 

 the extremities of the thoracic axial segments. Com- 

 pared with P. ( Portloclfia ) orestes Billings, the 

 basal lobe and tubercle are much more reduced, the 

 thoracic-axial tubercles are more strongly developed 

 and the axial segments relatively more slender. The 

 glabella is somewhat depressed, with first pair of side 



furrows short, weakly impressed, and come anter- 

 iorly. The second pair are short, weakly impressed, 

 and almost straight. The pygidium has four pairs of 

 ribs on the pleural lobes, divided distally by a 

 median sulcus. The fourth pair are faint. The 

 axis of pygidium is divided into about eight rings. 



Dorsal shield width 14 m.m., length 20 m.m. 



Cephalon " 14 m.m., " 6 m.m. 



Pygidium " 9 m.m., " 5 m.m. 



Horizon and Locality. Rare in the Ross Brook 

 formation, Arisaig, N.S. 



Collections. Victoria Memorial Museum, Yale 

 University collections. 



CONCLUSIONS. 



The results of the foregoing discussion may now 

 be summarized. 



(I). A generalized primitive line ranging from 

 the Ordovician into and through the Silurian is 

 recognized. The genus Dalmanitina Reed is ex- 

 tended to include all of these unmodified forms. 

 Thus interpreted it is considered to be practically 

 synonymous with Salter's use of Acaste Goldfuss 

 (non Leach) and in a broad way with Acaste as 

 used by Goldfuss. 



(2). A constant tendency to partial fusion of 

 the two anterior lobes and the frontal lobe, but 

 without the profound mcdification of the subfamily 

 Phacopinae Reed, is observed. Such Devonian 

 forms are recognized in the species for which J. M. 

 Clarke has proposed the generic name of Phacopina. 

 But these forms are not Phacopinid and exhibit none 

 of the profound specialization of the cephalon as 

 seen in that subfamily. Slightly modified species of 

 a similar nature are recognized in both the Ordo- 

 vician and Silurian but are thought to be evolutions 

 independent of Phacopina Clarke. They are pro- 

 visionally left under Dalmanitina. Another slight 

 modification is shown by some forms in which the 

 glabella is broadly expanded anteriorly, but which 

 retain the primitive pentamerism of Dalmanitina. 



(3). The subfamily Phacopinae Reed became 

 differentiated at the very beginning of Silurian time 

 and thereafter remained entirely distinct from the 

 primitive generalized line. The profound modifica- 

 tions cf this subfamily have been described by Reed 

 and have been reviewed above. Probably the most 

 diagnostic is to be found in the great reduction of 

 the third pair of glabella lobes and the tuberculation 

 of their extremities. Two stocks of this subfamily 

 are of present interest. 



(4). One of these Phacopid stocks is of wide 

 distribution in the Silurian and continues into the 

 Devonian, giving rise there to Phacops s. str. The 

 more primitive and ancestral Silurian forms of this 



