2 BRITISH FOSSILS. 



small spine. Hypostome with the centre gently swelled, and with dis- 

 tinct concentric furrows on the sides ; apex ? 



The first and last thorax rings are somewhat narrower than those of 

 the middle ; the axis is widest at top, and there only a little narrower 

 than the pleurse. It is not arched, but flattish, and marked by indis- 

 tinct tubercles on the sides and middle. The pleurae are straight ; the 

 fulcrum indistinctly marked about half-way along them ; the divisions 

 between the segments not nearly so conspicuous as the oblique straight 

 furrow which reaches nearly to the end of each. The ends of the 

 pleurae are recurved and pointed, the foremost ones perhaps not quite 

 so strongly as in our larger figure, the hinder ones as much as in fig. 2*. 



Tail a semicircle, with the upper angles rounded off; the axis hardly 

 more than two-thirds the length, flattened, conical, the end a little 

 pointed. There are about five distinct ribs on the axis, each obscurely 

 trituberculate. Lateral furrows four or five, very oblique, the two 

 uppermost sharp and deep, reaching more than half across the side, with 

 a secondary one between them, the two or three last very short and 

 faint. Margin flat. Whole surface of tail crossed obliquely by lines 

 which all run down from the upper angles inwards, except near the 

 edge, where they are concentric ; the incurved portion of the tail con- 

 centrically striate, broad, and indented by the axis. On the pleurae 

 the lines are longitudinal, and form a broad band crossing the segments. 

 We do not know their direction on the head ; the hypostome, however, 

 is strongly marked by raised concentric threads. 



Variations. In the young state the middle lobe of the thorax and 

 tail is narrower (of the latter considerably so), and therefore longer 

 proportionally than in the adult ; thus, in our smallest specimens, 

 fig. 3, the pleura? are once and a half as wide as the axis ; in fig. 4, 

 which is older, nearly as wide in proportion. In this stage the tail axis 

 is not visibly annulated, but the number of lateral ribs is the same as 

 in older specimens, about four ; the upper one is not, however, dupli- 

 cate. The glabella in the youngest specimens is very obscurely lobed, 

 and the eyes are thrown even further forward, close to the very narrow 

 border in front of the glabella. The latter is not incurved in front. 



The most interesting point about these young Oyygice is, that they 

 appear to confirm the supposition of the number of segments increasing 

 with age. Fig. 3, the youngest, has but four segments to the thorax, 

 and there is no appearance of displacement, the head is not shifted at 

 all. In fig. 4, a little older, there are seven rings, nor can we think 

 there is displacement in this case. The tail has turned a little, as on a 

 pivot, pushing the front edge on one side a little under the last ring, 

 but it has opened from that ring on the other side. Had a ring been 

 shifted off, and lost in the stone, we might have expected to see an 



