The Invertebrate Fauna of the Uitenhage Series. 67 



a much more complex nature. If, in a suitably preserved adult 

 specimen, the surface half-way down the valve and below this be 

 examined under slight magnification and with favourable illumina- 

 tion, it is seen that the raised interspaces or minute flattened ribs 

 themselves exhibit a peculiar punctate structure on that side of the 

 rib nearest to the lateral margin of the valve. This is developed in 

 such a way that the outer margin of each little rib is cut into by a 

 series of minute wedge-shaped indentations, which leave more 

 prominent projecting portions of the rib standing out somewhat 

 after the manner of the hydrothecae on a graptolitic stipe. In some 

 cases the successive wedge-shaped grooves may almost cross the rib, 

 but for the most part the margin of the rib which is nearest to the 

 middle line of the valve is straight and entire. 



Another peculiarity of the ornamentation is seen in some cases in 

 the manner in which, when the radial striae are traced upwards from 

 the valve-margin towards the place of convergence at the middle 

 line, by favourable illumination they may be observed to be continued 

 upwards across the middle line, giving rise to a slight intercrossing 

 here. 



In the left valve the anterior ear is large and rectangular, the 

 posterior ear small. This valve has a rather more equilateral 

 aspect than the right valve, which is sometimes very markedly 

 inequilateral. 



Dimensions. (1) (2) (3) 



Height 13 . 19 21 mm. 



Length 12 . 17 20 



No. (3) is Tate's figured type specimen. 



Occurrence. Eailway cutting between milestones 24J-24J on the 

 Uitenhage to Graaff-Keinet railway (297, 350) ; also obtained by Miss 

 M. Wilman at Coega Eiver. Tate says : " Collected by Dr. Eubidge 

 in a soft, yellowish-grey, sandy limestone at the Zwartkop Eiver 

 Heights, and at Prince Alfred's Eest ; in both localities it is 

 frequent." 



Remarks. The notched sculpture of the ribs, described above, 

 may be well observed when the preservation is exceptionally good. 

 It appears only in faint traces in a portion of the surface of one 

 of the specimens examined by Tate, and in other individuals less 

 well preserved the markings only consist of radiating linear strise, 

 obscurely punctate in places, with smooth interspaces. A right 

 valve from the Coega Eiver, which agrees closely with Tate's figured 

 type, has the surface so well preserved that the intimate details of 

 sculpture exhibited call for the above additional descriptive note. 



