BRITISH FOSSILS. 29 



the other hand, is very convex, while its posterior half is produced 

 into an abrupt conical protuberance, behind which it passes into a 

 broad plate, with a free, convex, posterior edge. This projecting 

 mass developed from the antennary sternum appears to me to repre- 

 sent both the epistoma and the labrum of decapod Crustacea. 



Viewed from below (Plate XVI. fig. 2) the part described has the 

 aspect of a very broad, almost triangular plate, occupying nearly the 

 whole width of the antennary sternum, and presenting a prominent 

 median portion and two lateral ate, which slope away backwards 

 and upwards. The posterior portion of the median lobe is rounded 

 and somewhat expanded at its extremity, and the lateral edges of 

 the expanded end are provided with fine seta3, which extend over 

 its superior face for a short distance. This conjoined epistoma and 

 labrum forms the anterior boundary of the oral vestibule; its inner 

 face is beset with short setse, and a short median tooth, rounded at 

 its extremity, projects backwards in the middle line. 



Posteriorly, the oral vestibule is bounded by a quadrate plate, the 

 nietastoma, directed forwards and downwards, and having about 

 two-thirds of the width of the labrum. As the sterna of the two 

 somites which immediately succeed the mouth, viz., the inandibular 

 and the first maxillary, are not distinctly separated from one another; 

 it is difficult to say from which thin plate arises. I am inclined to 

 think, however, that it is really a production of the maxillary 

 sternum, as its posterior boundary lies behind the level of the 

 anterior margin of the base of the maxillse. The free anterior extre- 

 mity of the plate is deeply divided, by a wide median excavation, 

 into two lobes, which are provided with many short and fine setae. 

 A little, accessory, setose, lobule is also developed from the inner 

 surface of the plate on each side. 



A well-developed endophragm separates the sternum of the first 

 maxillary, or second post-oral, somite from that of the rest. From 

 the mouth to the posterior edge of this, the third post-oral somite, 

 the sternal surface slopes gradually downwards ; but beyond this 

 point, or in the fourth post-oral somite, the sternum makes an abrupt 

 projection downwards and then passes backwards, with a general 

 parallelism to the axis of the body, for the rest of the extent of the 

 cephalo-thorax. 



The antennse are about as long as the proximal nine or ten joints 

 of the antennules. They consist of a short basal portion supporting 

 two branches of about the same length. The basal portion is three- 

 jointed, the inner branch two -jointed, the outer multiarticulate. 



