74 G. F. MATTHEW: ILLUSTEATIONS OF 



Some pyffidia having an axial lobe similar to that of this species and occurring with it, 

 possess marginal cusps. This species appears to be smooth externally, but is gramrlated 

 on the inner surface of the test. 



Sculpture. This species resembles A. gibbus (Linrs.), var. hybridus, Briig., but differs iu 

 having an elongated ridge on the front of the posterior lobe of the glabella, in the trans- 

 verse furrows on the cheeks, and in other respects. 



A. Richmondensis, Walcott, of the Prospect Mountain group, has affinities with our spe- 

 cies, but differs in the smaller lateral lobes at the base of the glabella, and in other 

 respects. 



Length, 13 mm. Width, 5 mm. 



Horizon and Locality. Freqiient in the fine dark shales of Div. l.d, at Porter's Brook, 

 St. Martin's. 



MICRODISCUS, Emmons. 



MiCRODlscus PUNCTATUS (Salter), var. pulchellus, Hartt. (Plate YII. Figs. 12a, b and c.) 



Microdiscm pulchellus, Hartt, mss. U. S. Geol. SurA'., Bull. 10. p. 25, PI. II. Figs. 1, 

 la, b and c. 



The following particulars may be added to the description of this form as given in 

 Mr. Walcott's Bulletin, No. 10 :— 



The short, transverse furrows on the marginal fold of the head shield differ from those 

 of Microdiscus Davsoni, in being closer together, and at about erj^ual distances all round; 

 these furrows do not always appear on the outer surface, but are distinct on the inner 

 surface of the test. The short furrow, which connects the dorsal and marginal furrows iu 

 front of the glabella, is faintly contiuu.ed across the anterior marginal fold. 



The occipital spine is much more slender iu my examples than in those of the Hartt 

 collection, figured by Mr. Walcott ; the spine is flattened vertically, and is often seen to 

 rise from the occipital ring at an angle of about 45° from the plane of the head shield. 



In the pygidium the median lobe has, as Mr. "Walcott says, eight segments, including 

 the articulating ring ; but beside these there are three narrower and less distinctly marked 

 segments at the extremity, which would not be observed in heads that are badly preserved. 

 When it is one quarter grown, this variety has only seven of the large rings (including 

 the articulating ring) in the pygidium ; this is the number which, according to Mr. Salter, 

 marks (the adult of) Blicrodiscus punctatus. Considering that the Acadian form has ten 

 segments in the median lobe of the pygidium, and bearing in mind the other points of 

 difference noted by Mr. Walcott, it seems entitled to rank as a distinct variety of the 

 European M. punctatus. 



Mr. Walcott's measurements give nearly the extreme size attained by our variety, and 

 more frequently it is one third smaller ; it occurs in immense numbers in certain beds of 

 of dark grey shale in the eastern part of the St. John Basin. 



Length of head-shields (without the occipital spine), 4J mm. Width, 5 mm. ; length of 

 occipital spine, 3 mm. 



Length of pygidium, 4J mm. Width, 5 mm. There is considerable variation in the 

 proportion of the parts. 



