SCH1ZOPORELLA HYAL1NA. 273 



deep and narrow. The extreme forms (the simply orbi- 

 cular and that with a subtubular sinus) are connected by 

 intermediate varieties. In many cases the two projec- 

 tions or denticles, placed one on each side of the aperture 

 a little above the inferior margin, which mark the posi- 

 tion of the hinge of the oral valve, are quite incon- 

 spicuous, and the mouth appears almost perfectly cir- 

 cular. In other cases they are more strongly developed, 

 and stand out so prominently as to produce a decided 

 narrowing of the aperture, while, at the same time, they 

 define a broad and shallow sinus below. In this con- 

 dition there is only a slight departure from the orbicular 

 type ; but a further change in the same direction brings 

 with it the contraction and deepening of the sinus, and 

 affects materially the contour of the mouth. 



A common variety (cornuta) (Plate XLV. fig. 2) has two 

 erect processes, rising one on each side of the orifice. 

 But perhaps the most remarkable deviation from the 

 ordinary type is met with in an Australian form, which, 

 in spite of its peculiarities, must be referred to this 

 species. On each side of the orifice, which is furnished 

 with a deep and narrow sinus, is a process such as we 

 find in the variety cornuta. Immediately below the infe- 

 rior margin rises a very tall mucro, broad at the base, and 

 tapering upwards, which usually bends inward over the 

 mouth. A line of prominent tubercles runs down each 

 side of the cell on the front wall ; and frequently one is 

 also present in the centre. The ovicell is developed on 

 the ordinary zooecia, is flattened in front, with a number 

 of tubercles round the edge, and an umbo in the middle. 

 Notwithstanding the extraordinary development of tuber- 

 cular processes in this form, it agrees so fully in essential 

 characters with S. hyalina, that it is properly ranked as a 

 variety. 



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