FROM THE LIAS. 145 



Hemipkdina Bowerbankii, Wright. PL IX, fig. 2 a, h, c. 



Hemipedina Bowerbankii. Wright, Annals and Magazine of Natural History, 2d series, 



vol. xvi, p. 96. 



Ambulacral areas narrow, with two rows of marginal tubercles, rather smaller than 

 those of the inter-ambulacra, one tubercle on ever}' third or alternate plate ; inter-ambu- 

 lacral areas wide, with six rows of tubercles abreast, each surrounded by a delicate areolar 

 circle ; spines long and needle-shaped, deeply sculptured with longitudinal lines. Form 

 unknown, as the test is crushed. 



Dimensions. — One inch and one twentieth of an inch in diameter; height unknown. 



Description. — The only specimen of this species I have seen is the one collected by my 

 friend Mr. Bowerbank, and figured in PI. IX, fig. 2. The test is unfortunately much 

 crushed, but a suflacient number of plates are preserved to enable me to make a short 

 description of this beautiful Liassic form. 



The ambulacral areas are more than one third the width of the inter-ambulacral ; they 

 are composed of narrow, hexagonal plates (PI. IX, fig. 2 b) ; on every third plate a small 

 tubercle is developed, about midway between the zones and the central suture, but nearer 

 to the latter ; on the intervening plates there are only two or three small granules on each ; 

 the poriferous zones are narrow and straight, the pairs of pores having an oblique" direc- 

 tion upwards and inwards ; there are four ambulacral plates opposite each inter-ambulacral 

 one ; the number of plates in a column cannot be ascertained, as all of them are more or 

 less incomplete. 



The inter-ambulacral areas are more than double the width of the ambulacral ; they 

 are composed of narrow, pentagonal plates (fig. 2 /5) ; on the centre of each is a primary 

 tubercle, and on each side a smaller secondary tubercle ; these three tubercles are disposed 

 nearly on the same line, and two smaller tubercles are developed near the lower angles 

 of the plate ; the central tubercle is surrounded by an areola, and an incomplete circle of 

 small, spaced-out granules encircles it ; granules of the same size form circles around 

 the secondary tubercles, but in them the areola is small or altogether absent. 



At the equator each inter-ambulacra is provided therefore with six rows of tubercles, 

 which are arranged nearly all abreast, and four rows of smaller tubercles placed near the 

 lower angles of the plates ; but in the upper part of the area, the rows of secondary 

 tubercles dwindle into granules, and finally disappear (fig. 2 a) ; there are four pairs of 

 pores opposite each large inter-ambulacral plate (fig, 2 b). 



The apical disc is not preserved ; and as the base is imbedded in the clay, the structure 

 of the under surface is unknown. 



19 



