160 PELTASTES 



The poriferous zones are narrow, the pores small and placed in oblique pairs, of which 

 there are forty in each zone. The septa between the pores are capped by a prominent 

 granule. 



The inter-ambulacral areas are wide, and the plates in the columns of unequal size. 

 There are three large tubercles in each area ; of these one is very large, and two mode- 

 rately so ; the five others diminish gradually towards the peristome ; the upper surface is 

 destitute of tubercles, and this vacant space is covered with small, various sized, sparsely 

 scattered, mamniillated tubercles ; a series of these encircle the areolas of the primary 

 tubercles. Eig. 2/ represents an inter-ambulacral area, magnified four diameters, 

 showing the great disparity existing between the volume of the different tubercles deve- 

 loped therein. The miliary zone is likewise ornamented with similar bodies. Fig. 2 h 

 displays these parts, magnified six times, in a single plate, and fig. 2 2a tubercle seen 

 in profile. 



The base is wide and flat, and highly ornamented with the small close-set tubercles 

 of the inter-ambulacral areas. The oral aperture, one third the diameter of the test, lies 

 in a depression (fig. 2 c). The peristome is divided into ten lobes by feeble incisions. 



The apical disc occupies a large portion of the upper surface ; its border is exceed- 

 ingly sinuous, and deeply incised at the marginal boundary line between the ocular and 

 ovarial plates ; from each oviductal hole two pairs of elliptical-shaped incisions proceed, 

 and right and left to join those proceeding from the adjoining oviduct ; and these form 

 the sides of the pentagon which extends around the disc. The sur-anal plate is marked 

 by two pairs of elliptical incisions, which join those from the anterior ocular, and thus 

 form a diamond -shaped impression before the vent (fig. 2 c). The ovarial plates termi- 

 nate in three-lobed cardiform expansions, and from each side of the orbits similar heart- 

 shaped trilobate bodies stretch obliquely outwards. The disc of this species forms a 

 most complicated structure, of which words fail to give any adequate idea. Fortunately, 

 my friend Mr. Bones' admirable figures supply the eye with the fine symmetrical propor- 

 tions of this intricate bit of anatomy. Consult fig. 2 a, b, d, e, for all details. 



The vent is large and looks obliquely backwards ; it is of a diamond shape, with a 

 thick prominent periprocte ; at the three posterior angles of this bourrelet the single and 

 postero-lateral oviducts open, the two antero-laterals are placed much in advance on the 

 lateral boundary of the sur-anal. 



Affinities and Differences. — The sculpture of the apical disc and the deep incisions 

 through the same reminds us of some forms of Peltastes dathratus ; the margin, however, 

 is more deeply indented, and the style of ornamentation on the same is diff'erent ; the 

 marginal tubercles on the ambulacra are smaller and more remote, and divided by a 

 prominent granulated ridge ; the structure of the inter-ambulacra exhibits a considerable 

 difference, and the pores in the zones are smaller and more numerous. 



Locality/ and Stratigrapldcal Position. — This rare Salenia was collected from the 

 Lower Chalk, Mildenhall, near Bury St. Edmunds. The original and unique specimen 

 is in the Museum of the Royal School of Mines. 



