FROM THE WHITE CHALK. 277 



Dimensions. — Length three inches ; breadth two inches and seven twelfths ; height 

 one inch and nine twelfths of an inch. 



Description. — My late esteemed colleague in this work, Professor Edward Forbes, 

 gave the following note upon this Urchin in Decade III of the ' Memoirs of the Geological 

 Survey,' in his note on allied species of British species of Micrasters : — " I have given an 

 account of it in Dixon's work on the ' Geology of Sussex,' where it is excellently figured. 

 I have there named it Micraster cor-bovis. It is usually a larger and longer species than 

 cor-anguinum, and its petaloidal ambulacra are more deeply impressed and much shorter 

 in proportion to the body. The shape is ovato-cordate, the curve of the sides from the 

 front of the antero-lateral ambulacra to the anal extremity being but slight, its chief swelling 

 being near the anus, and not on a line with the postero-lateral ambulacra as in cor- 

 anguinum. The back is more equally depressed than in the depressed variety of the last- 

 named species. The mouth is much smaller comparatively, and the post-oral spinous space, 

 though much longer, in consequence of the elongation of the hinder portion of the test, is 

 nevertheless proportionally broader. The tubercles of the plates, whether dorsal or 

 ventral, are much smaller and more scattered. Besides all these comparative characters, 

 there is the positive one that in cor-bovis the ambulacral plates, instead of being tumid, 

 are smooth and plain, as are also the ridges separating the sulcations of the pairs of pores 

 in the petaloidal ambulacra. The ambulacral spaces are wider than the breadth of any 

 of the sulcations." 



It will be more satisfactory if I here insert Forbes' note from the work referred to : 



" The body of this fine and large Micraster is ovate and slightly cordate, broadest 

 in the region of the antero-lateral ambulacra. The posterior end is obtusely subtruncated. 

 The dorsal surface is depressed and but slightly elevated in the anterior region above the 

 rest of its surface. 



" The ovarian circle is placed nearer the anterior than the posterior end. The frontal 

 groove is shallow. The lateral ambulacra are placed in gentle depressions. 



" The postero-lateral ambulacra are very short, and little more than half the length 

 of the anterolaterals. There are about thirty pair of pores in each row in the latter and 

 about seventeen in each row in the former. The larger tubercles of the dorsal plates are 

 much scattered and minute in proportion to the size of the shell. The interstices are 

 minutely granulated. 



" The areolated tubercles of the ventral surface are also proportionally small. The 

 post-oral spinous space is triangularly lanceolate." 



Affinities and Differences. — This species resembles M. breviporus, of which it has long 

 appeared to me to be a gigantic variety. 



Locality and Stratigraphical Position. — Dixon's type was collected from the White 

 Chalk, Sussex, others are found at Charing, Kent, and some fine specimens in the 

 Woodwardian Museum, Cambridge, were obtained from Balsham, near Cambridge ; it 

 occurs low down in the White Chalk. 



