550 NEW BKACIIIOPODS PBOM THE MIDDLE PALAEOZOIC ROCKS, N.S.W., 



valve ; but from that to the adult stage the umbo and umbonal regions strongly 

 develop; and so throw the pedicle valve beak on to that of the brachial valve. 



In the classification of this brachiopod 1 have adopted the divisions proposed 

 by Hall for galeatiform pentamerids (Pal. N.Y., Vol. viii., Brach., ii., 1894, pp. 

 240 and 247). 



Loc. and horizon. — Hatton's Corner, Yass River, Parish of Hume, County 

 Murray, associated with Barrandella [Clorinda) linguifera var. wilkinsoni Eth. 

 Junr., Atrypa reticularis Linn., Hhiiopht/llum interpwnctalum de Kou., Kncrinurus 

 mitchelli Foerste, etc. Uj^per Silurian (Wenlock). 



Family ORTHIDAE. 



Orthis ( Schizophoria ) striatula Sowerby. (Plate xxxi., figs. l('-20.) 



Spec. Chars. — Outline subcircular or subelliptic, transversely biconvex, sur- 

 face densely covered with fine radial striae which increase in number anteriorly 

 with the growth of the shell by dichotomy and occasional interpolations; at in- 

 tervals the larger striae open to the surface and discontinue. All the striae along 

 their whole length are surmounted by slight asperities ; concentric growth lines 

 faint except anteriorly. Pedicle valve distinctly convex except anteriorly, where 

 it becomes depressed, and in senile shells a distinct sulcus is formed ; beak only 

 slightly incurved and higher than that of the brachial valve. Brachial valve more 

 convex than pedicle, beak incurved. Cardinal area of moderate length, triangu- 

 lar, elevated in each valve. Delthyrium conspicuous. Cardinal angles rounded. 

 Anterior margin in immature shells very mildly sinuate; rather strongly in some 

 of full growth . 



Dimensions (mature and nearly mature specimens). — 



Length 20.3 mm. Width 23.4 mm. Depth 15.6 mm. 



20.3 „ 23.3 „ 12.5 



17.2 „ 22. (i „ 11 



„ 17.2 „ 22.7 „ 12.5 



20.7 • „ 25.0 „ 12.5 



These measurements show proportionate relations more or less, for the three 

 dimensions. The first, which has the greatest thickness, has also the appearance 

 of greatest age; it would appear that depth continued to increase after the other 

 dimensions had reached their full development. 



Ohs. — Some ])alaeontoIogists have contended that O. (Sch.) striatula, is 

 idiMitical with (>. (iSch.) resupinata; but a larger number recognise its specific 

 rank. The Australian representatives of the species are of smaller size than the 

 European and North American forms; but agree with them in external features. 

 The local O. {Sch.) striatula has only half the width and length of the local 0. 

 (Sch.) resupinata Martin, but in depth often exceeds the latter, in the case of full 

 grown specimens, ami is mu(^h more convex. In no instance have I noticed the 

 anterior marginal sinus so pronounced in the latter as it is in the former when 

 the shells are of mature growth. The muscular scars of the former, as far as 

 my observations have enabled me to decide, are less distinct than are those in the 

 latter, and in other respects the scars appear to differ. The local fossil seems 

 nearer in form and dimensions to the North American form than to the British 

 one. 



The specimens here dealt with occur in association with Merista plebeia 

 Sowerby; and in th:\l respect agree with the European and North American a.sso- 



