MICBODIS.CUS PUNCTATUS. 37 



widest in front, somewhat narrower at the sides, slightly produced at the genal 

 angles ; smooth on the outer surface, marked on internal casts by close-set 

 transverse impressions which divide it into a row of beads. 



Thorax of three segments. Axis narrow, only about half as wide as the 

 pleurae. Pleurae straight, bent downwards at the tips, with a broad groove ; 

 pointed ; the points in the first segment directed backwards, in the other two 

 segments forwards. 



Tail semi-elliptical, with the anterior angles truncated, marginate. Axis 

 narrow, forming about one fifth of the total width in front, tapering gradually, 

 divided into a number of rings, of Avhich eight or nine are usually distinct, 

 reaching nearly to the posterior margin, separated from the lateral lobes by very 

 deep axial furrows. Lateral lobes with a broad, deep groove in front, but other- 

 wise without furrows or ribs, wide in front, very narrow behind the axis, where 

 they are confluent with each other ; test perforate, or punctate both externally and 

 internally. Margin narrow, nearly equal in width throughout. 



Head- and tail-shields about o mm. long and 4 mm. broad in specimens of 

 average size. 



There are important differences between the exterior and the interior of the 

 test of this species, and consequently the external moulds differ in several respects 

 from the internal casts. These differences are well shown in the series of speci- 

 mens in the Sedgwick Museum and in other collections. In the moulds which 

 show the exterior of the test, the furrow in front of the glabella (which appears in 

 the mould as a ridge) is a narrow and not very strongly impressed line; in the 

 internal casts it is a deep and broad groove. In the actual tests there must 

 therefore have been a strong interior ridge in this position. In correspondence 

 with this difference the cheeks in the moulds appear to be nearly of uniform 

 width throughout, Avhile in the casts they narrow forwards. In the external 

 moulds the impressions on the sides of the glabella are less distinct than in the 

 internal casts. The margin of the head in external moulds is smooth, in internal 

 casts it is crenulate. The test of the head was finely punctate external^ ; in 

 internal casts the surface is nearly smooth. The test of the tail was either 

 perforate or punctate both on the exterior and interior surfaces. External moulds 

 therefore show small eminences projecting inwards, indicating that the tail was 

 punctate, while internal casts show similar eminences projecting outwards, 

 suggesting that the tail was tuberculate. In both cases, and especially in the 

 internal casts, the ornamentation is often indistinct and the surface nearly 

 smooth. 



In young forms the lateral impressions on the sides of the glabella are much 

 more strongly marked than in the adult, and distinct lateral lobes may sometimes 

 be recognised. The neck-spine also is shorter, and there are fewer rings on the 

 axis of the tail. 



6 



