MITRASTER. 57 



line are mucli larger and higher than the other plates. The plate adjacent to the 

 median interradial line measures about 4 mm. in length and 4 mm. in height ; 

 the next plate 5 mm. in lengtli, and from 3"5 mm. to 3'75 mm. in height. The 

 surface of the plates is marked with small widely spaced punctatious, and there is 

 a narrow depressed border round the entire margin of the plate, which is very 

 finely punctate. 



The actinal area is unknown to me. 



Hisfori/. — The fossil delineated on PI. XIV, fig. 5 a, was drawn by Mr. A. H. 

 Searle under Dr. Wright's instructions, but I regret that I have not been able to 

 find any trace of the specimen. I am therefore led to believe that the type 

 belonged to Dr. Wright's private collection, which has been distributed since his 

 lamented death. Knowing by experience the extreme care and fidelity which 

 characterise all Mr. Searle's work, I have ventured to describe the species from 

 his drawings alone, for it seemed undesirable to leave such an interesting form 

 without notice ; and I am hopeful that the publication of the figure and the 

 description of its characteristic features will lead to the detection of the type. I 

 am unable to give any information as to the locality or stratigraphical position 

 from which the fossil was obtained. 



Bemarlca. — The rather small size of this example and the small number of 

 supero-marginal plates — only four for the whole side of the disk — would not 

 unnaturally suggest at first sight that this was possibly an immature form. After 

 careful study, however, I do not consider such to be the case, or at any rate I am 

 unable to regard the fossil under notice as the young of any of the species with 

 which I am acquainted. The large and characteristically developed ultimate 

 supero-marginal plates in conjunction with the presence of a normal number of 

 infero-marginal plates, together with the fact that the size of the test is not less 

 than that of another perfectly characterised species, lead me to rank this as a 

 distinct species with little hesitation. The general proportions as well as the 

 character of the different plates, and the facies of the form as a whole, appear to 

 me to fully warrant this view. 



Gemts— MITRASTER, Sladen. 



(iliTpa = a broad belt, or girdle.) 



Body depressed and cycloid, or cyclo-pcntagonoid in contour. Marginal plates 

 with co-ordinated granulose elevations and punctations, and a surrounding narrow 



