METOPASTER QITADRATUS. 97 



mostly collections of isolated plates. The specimen registered 35498 is taken as 

 the type. 



Description. The abactinal surface of the disc appears to have been covered 

 with a number of plates of generally uniform size, with an average diameter of 

 about 3 '8 mm. A few plates exceed this size, but in no case are they as large as 

 the corresponding plates in P.punctatus. 



No specimen is sufficiently well preserved to give the proportionate lengths of 

 the major and minor radii, but there is no doubt that the arms were considerably 

 produced. A row of hexagonal tabulate radialia are present throughout the greater 

 portion of the arm. The breadth of the arm at the base in the specimen registered 

 E. 2576 is about 22 mm. This specimen, however, judging by the dimensions of 

 the marginal plates, does not by any means appear to have attained the usual size 

 of the species. The length of its minor radius is 18'5 mm. 



The median supero-marginalia are quite smooth and are distinguished from 

 those of all other species of the genus by their thickness (Text-fig. 24 ). 

 In full-grown specimens they appear to be 18 mm. in breadth, 6 mm. in length, 

 and 10 mm. in thickness. More distally the supero-marginalia acquire large 

 mammilate rugosities. 



The infero-marginalia are similar in character to the superior series. 



Two rows of ventro-lateralia enter the base of the arm. Most of the ventro- 

 lateralia appear to have been rhomboidal in shape and of uniform size. They 

 possess very shallow hexagonal fittings indicating the former possession of 

 granules. 



Locality and Stratigraphical Position. Upper Chalk, Kent. 



Genus METOPASTER, Sladen. (See p. 30.) 

 9. METOPASTER QUADRATUS, n. sp. Text-figs. 1, 2, p. 98. 



Specific Characters. Marginal plates in interbrachial areas almost square. 

 Raised area on marginal plates without spine-pits. Supero-marginal plates rugose 

 on interior surface. Ultimate supero-marginal plates may or may not be the 

 largest of the series, variation in this respect being especially marked. Abactinal 

 plates of disc with distinct stellate marking. 



Material. There are three fairly perfect and four fragmentary specimens of 

 this species in the collection of Dr. Blackmore, of Salisbury. Two of these are 

 figured in Text-figs. 1, 2. The species was discovered by Dr. Blackmore, who 

 suggested the specific name " quadratus " on account of the characteristic shape 



