176 Dr. T. Wright on the Cidaridae of the Oolites. 



dismal plates is covered with the same delicate granular sculpture 

 which adorns the intertubercular parts of the test. The ventral 

 surface is very concave; the mouth-opening is large, about half 

 the diameter of the test, situated in a considerable concavity 

 formed by the ambulacral and interambulacral arese curving up- 

 wards and inwards towards the interior of the test ; the margin 

 is decagonal, with nearly equal-sized lobes ; the teeth are cari- 

 nated ; the primary spines are cylindrical and tapering, and rather 

 exceed in length the diameter of the test ; their surface is sculp- 

 tured with longitudinal microscopic lines ; the secondary spines 

 are small, delicate, hair-like appendages. 



Affinities and differences. — A. decor ata belongs to the group of 

 Salenians having the anal opening situated behind the apical 

 disc; it consequently has affinities with A. spinosa and A. Wil- 

 tonii } which it further resembles in the rudimentary condition of 

 the single ovarial plate, and in the general structure of the am- 

 bulacral and interambulacral arese ; it is distinguished however 

 from A. spinosa by having a more pentagonal outline, a more 

 rudimentary condition of the dorsal tubercles, a more oblong and 

 irregular-shaped apical disc, with the single ovarial plate project- 

 ing further than the others into its corresponding interambu- 

 lacrum, and the sur-anal plate being formed of many elements 

 instead of one, as in A. spinosa. The ventral surface is likewise 

 more concave, the mouth-opening is proportionally larger and 

 lodged in a deeper concavity, and the marginal lobes are more 

 equal-sized than in A. spinosa. The same group of diagnostic 

 characters will serve to distinguish A. decor ata from its other 

 congeners. 



Locality and stratigraphical range. — This beautiful Acrosalenia 

 was collected in the yellow clays of the Coralline Oolite of Wilts, 

 and I have seen it in the ragstones of the same stage. The 

 test is well preserved in the individual which served for our 

 description, but the apical disc is unfortunately absent. Our 

 description of the disc is given from another specimen. 



Acrosalenia TViltonii, Wright, n. sp. PL VII. fig. 4«-e. 



Test hemispherical, sometimes depressed, sides tumid ; ambu- 

 lacral area? narrow, with two rows of small marginal perforated 

 tubercles ; interambulacral arese about three times the width of 

 the ambulacral, with two ranges of primary tubercles, of 

 which the three middle pairs only attain full development; 

 those at the base are small, and those at the dorsal surface are 

 rudimentary ; apical disc convex and prominent ; sur-anal plate 

 formed of two large and five small pieces ; anal opening be- 



