THE FAUNA OF THE ST. JOHN GROUP. 37 



Externally, the surface of both valves is marked by fine radiating striae, which are 

 crossed by more distinct concentric stria\ The surface of the ventral A'alve (as preserved 

 in the shales) has also a number of concentric ridges. 



The dorsal valve of this species is not unlike that of Linnarssonia transversa, but the 

 two species are easily distinguished by their ventral valves, and they difl^er much in the 

 thickness of the shells; in Acroireta Bailei/i the shell is A^ery thin, so much so that in the 

 dark sandstones, near the base of Division 1, where apparently it is first found, it can be 

 recognized as an Acrotreta only by the peculiar form of the A'eutral valve. This species in 

 its form is like A. Nichohoni (Davidson), of the British Cambrian strata ; it may be distin- 

 guished from that species by the internal markings of the dorsal A'ah^e. Our species is 

 evidently quite distinct from A. sodalis (Von Seebach), which has a much higher ventral 

 valve, and has been found at several horizons in the Parodoxides beds of Sweden. 



Length 3| mm. Width, 4 mm. 



Horizon and Locality. In the dark, sandstones of Div. \.h', at Hanford Brook are 

 distorted examples of an Acrotreta which may be this species. The species occurs at 

 Long Eeach, King's County, with Ptychojxirin Robhi (?) in shales, the exact age of which 

 has not been determined. At this locality it was collected by Professor L. W. Bailey. 



AcROTKETA (?) GuLiELMi, u. sp. (Plate V. Figs. 14, 14«, b, c,d and e.) 



Shell substance corneous. Outline circular, or slightly wider than long. 



The dorsal valve is saucer-shaped, being slightly convex, and haA'ing the margin 

 involute. This A'alve has an outer zone of growth that is apparently thicker than the 

 central part, as the latter is often only imperfectly preserved. The position of the umbo is 

 somewhat obscure, but appears to be about one quarter or one fifth of the diameter of the 

 A'ah'e, from its posterior side. 



The interior of the dorsal A'alve, in the examples known, has an oA'al or circular scar, 

 extending from a point about one fifth from the posterior side to the middle of the A'alA'e. 

 Outside of the central scar, the surface is divided into four segments of unequal size by 

 faint furrows extending to the margin. The tAVO posterior fuiTOws are nearly straight ; the 

 two anterior are longer and arch outward from the front of the central scar towards the 

 lateral margins. The posterior segment is smaller than the others, and there the A'ah'e is 

 deflected upM^ard (as seen Avhen looking doAvn into the valve) ; within this segment of the 

 A'ah'e there are tw^o minute pits on the posterior edge of the central scar, and behind these 

 are two minute (muscular ?) tubercles ; both pits and tubercles are close to the axial line, 

 which is marked hj a narrow, faintly-raised ridge, extending to the posterior margin of 

 the A'ah'e. Each lateral segment of the A'ah'e has a narroAA', faint ridge, AA'hich cuts otf a 

 slightly depressed, lunate portion on its anterior side. The anterior segment of the A^alve 

 has a median furroAV, proceeding from the front of the central scar, and forking about one 

 third from the front of the A'ah'e ; here it becomes faint, and encloses a shallow, OA'al depres- 

 sion ; within the anterior segment of the Aahe there are also tAVO short faint furroAvs, 

 one on each side of the median furrow, extending outward from the central scar. The 

 inner surface of the A^ah'e, on each side of the central scar, and in front of it, is coA'cred 

 with faint raised lines radiating to the margin, and similar radiations appear to be indicated 

 along the posterior border of the vah'e. 



