30 PEOCEEDINGS OE THE GEOLOGICAL SOCIETY. [NoY. 21, 



tween the Limulidos of to-day and the extinct Enrypteridse of Palaeo- 

 zoic times. 



In Pterygotus, Eurypterus (see PL I. fig. 7, Eurypterus remipes), 

 &c. the animal consists of a head, hearing a pair of minute larval 

 eyes and a larger pair of suhcentral or marginal compound ones, a 

 pair of simple or chelate antennae (PL II. fig. 9), furnished with 

 gnathites in Eurypterus and Slimonia (PL II. fig. 10), three pairs of 

 more or less slender spinous pedipalps, never chelate, and a pair of 

 very hroad and powerful swimming-feet (PL I. fig. 7, e), whose hasal 

 loints form powerful mandibles. In Stylonurus the two posterior 

 pairs of appendages are converted into long and slender rowing- 

 organs. 



PoUowing this compound head are twelve free segments, only the 

 first two (?) of which bear appendages (PL TI. fig. 8), the succeed- 

 ing somites being apparently destitute of any. The twelve free 

 segments are followed by a postanal plate or "telson," broadly 

 lanceolate or bilobed in Pterygotus and Slimonia, and ensiform in 

 Eurypterus and Stylonurus (PL I. fig. 7, t). 



In Lhnulus (PL II. figs. 1 & 2) the head is composed of a broad 

 buckler having the larval and compound eyes upon its superior or 

 convex surface, and the mouth, surrounded by six pairs of chelate 

 appendages, placed beneath the head-shield (PL II. fig. 6). The 

 first pair of appendages is placed in front of, and the four suc-- 

 ceeding pairs are posterior to the mouth. These latter are furnished 

 with spinous gnathites, the most posterior pair serving as maxUlce. 



ISText follows the operculimi or thoracic plate (PL II. fig. 7), which 

 is attached to the posterior margin of the head, and bears upon its 

 inner and upper surface the reproductive organs or ovaries (PL II. 

 fig. 7, r). 



The six anchylosed segments which compose the posterior portion 

 of the body bear five lamellar appendages upon their under surface, 

 similar in shape to the operculum, but more membranous ; these 

 support the gills, and are partially hidden beneath the operculum. 



The body terminates in a long ensiform '' telson," or tail-spine. 

 The anterior portion here spoken of as the head has always been 

 regarded as representing the cephalothorax, and the posterior por- 

 tion as the abdomen ; this latter I propose to call the thorax (in 

 part), and to give my reasons for so doing as we proceed. 



The great and apparent diff'erence between Limulus (PL II. 

 figs. 1 & 2) and Eurypterus (PL I. fig. 7) is, that in the latter one 

 sees fourteen free segments or divisions, whilst in the former only 

 three are visible. But if we examine Limulus attentively we can 

 still trace in the posterior portion of its shell (PL II. figs. 1 & 2) 

 indications of segmentation, whilst the presence of thirteen paired 

 appendages indicates the coalescence of numerous segments. 



The points of resemblance are : — the larval eye-spots ; the adult 

 suhcentral eyes; the jaw-feet, serving as the sole locomotive organs, 

 the basal joints in the posterior pair being especially fitted for 

 manducation, and the posterior limbs for swimming ; the analogous 

 position of the opercular plate, covering, no doubt, in the fossil 



