268 THE ATRVl'lUAi; Oi' NEW SOUTH WALES, 



Brachial valve very convex to gibbous in large specimens, laterally strongly turned 

 up. The spiralia are of the true Atnjpu type and, with cones, consist of ten or 

 more spirals with their apices directed towards the outward centre of the brachial 

 valve; apices of the cones are apart, but opposing sides compressed. Uimensiuns: 

 The lai-gest of specimens has a length and width of one and a-half inches without 

 the marginal fringe, and a depth of three quarters of an inch. This specimen, 

 with the fringe, would have had a width of not less than two and a quarter inches. 



Qlis. — The valves in young specimens are very mildly and about equally con- 

 vex, and the umbo, cardinal ridges and beak form an isosceles triangle, the base 

 of which is from one cardinal angle to the other, but with age the brachial valve 

 becomes more and more convex until, when full growth has been reached, it is 

 strongly s-o, or gibbous. This form resembles both A. reticularis and A. des- 

 quamata, but differs from each of them in the fineness of its surface ornamentation 

 and the strong curve of the radial striae which is directed laterally. From ^-1. 

 reticularis it differs in the high, erect, pointed beak of the pedicle valve, the more 

 exposed deltidium and false area, exceedingly fine concentric growth lines, and 

 much greater dimensions when compared with normal representatives of the 

 species. In the proportions of the leng-th to width, and also in contour it agi-ees 

 with A. reticularis. It resembles ,1. desquamata in having a high pedicle valve 

 beak, exposed circular foramen and deltidial plates, prominent umbonal ridges 

 and false area, also in the features of the brachial cones, but with some of these 

 resemblances it is merely a matter of degree, because the beak of the present 

 species is very much higher and erect, more acutely pointed and practically with- 

 out incurvation. The umbonal ridges are less divergent, higher, and with the 

 beak form a triano-lf with an acute apical angle, <|uite different from the triangle 

 formed by these ridges in ^-1. desquamata. Then there are the differences already 

 referred to — the much finer superficial ornamentation of the shells of the local 

 form, and the strong curving of the radial striae laterally and towards the cardinal 

 angles. The liinge line of A. desquamata is mildly arcuate, that of the other un- 

 dulating. Plainly our species resembles in several particulars both ^-1. reticularis 

 und A. desquamata, and the latter in more than it does the former. 



Considering the tendency to variation among the Atrypa group some might 

 be disposed to recognise A. erectirostris as a variant of A. desquamata, but it is to 

 be noted that, though the differences between the two forms in some instances 

 appear only to be of degree, V'et they are permanently estaljlislied ; for, though 

 the normal form of .1. desquamata is found in association witli tlic one hero de- 

 scribed, there is no evidence of gradation of the one form into the othi>r. and this 

 affords an additional reason for a separation of the two types. 



Loc. and horizon. — Cave Flat, near the jiuiction of the Goodradigbce with 

 tlie Murrumbidgee River, an area that is now submerged by the waters impounded 

 within the Bai-ren Jack (Burrin.jiick) dam, where it occurs i)lentil'ully in a stratum 

 'of limited thickness, associated with Spirifer i/assensis and many other Spiriferi 

 not yet determined, Iiht/ncli<iiiella, Atrypa desquamata, Ci/rtina, etc. Parish of 

 Woolgarlo, County of Harden. Probably lower Middle Devonian. 



Atrypa MARGiNALis Dahnan . (PI. xv., figs, l-l-lti; PI. xvi., fig-s. 1-5; Text-figs. 1-5.) 

 The following is Davidson's description of this species*: — 

 "Spec. Char. Transversely subpent agonal or almost elliptical: hinge line 



nearly straight; lateral margins rounded, very slightly so in front, beak of ventriil 



•Mon. Brit. Dev. aii.l Sil. I!i-iu-hio]ioaa. vul. iii.. Scot. Sit. Hracli., iii>.l.Ti-4. 



