1)0 BRITISH FOSSIL CRUSTACEA. 



The first or coxal joint (co) is the largest, measuring 9 lines in its greatest length 

 and 9 lines in its greatest breadth. The tongue-shaped extension (/) unites it to the 

 head and to the muscles by which the palpus (b — u) and the endognath (g) are moved. 



The mandibular portion (g) is armed with from 7 to 8 sharp and recurved teeth about 

 1 line in length. From the upper side of the endognath the palpus takes its rise : (b) the 

 basos measures 6 lines in length and four lines in breadth : (i) the ischium commences 

 4 lines in breadth, but contracts to 2 lines at its distal end ; it is 3 lines in length : the 

 four succeeding joints are nearly cylindrical in form : (m) the meros is 8 lines in length 

 and 2 lines in breadth : (c) the carpus is 3 lines in length and 2 broad : (p) the propodos 

 is 4 lines long and 1 ^ broad : (d) the dactylus is 9 lines long and 1 line broad : (u) the 

 unguis or terminal joint is 4 lines long and 1 line broad. 



The structure of this pair of appendages differs widely from the antennas in the pre- 

 ceding genus (see Parts I & II), and seems to indicate a close affinity with Eurypterus (to- 

 be presently described). 



Instead of the powerful chelate appendages with which the front of the head in Ptery- 

 gotus was armed, we find in Slimonia only this pair of minute and simple eight-jointed 

 antennas which could merely have served as feelers (and accessory endognaths), not as 

 organs of prehension. 



Another essential point of difference to be noted in these organs as compared with 

 those of Pterygotus is in the fact that the chelate organs in Pterygotus do not perform the 

 functions of manducation with their basal joint, but only subserve the purposes of prehen- 

 sion, and to convey food to the mouth, being placed in front of the buccal orifice. 



In Slimonia the small slender antennas are expanded at their base so as to form an addi- 

 tional pair of jaws (endognaths), and are, therefore, directly in relation with the buccal 

 orifice. 



These distinctions are of the highest importance in considering which of the two 

 pairs of antennary organs has been suppressed in the Eurypterida. 



If we take Pterygotus by itself, the fact of its antennas not being furnished with a 

 mandibular basal joint would lead to the inference that these great chelate organs of pre- 

 hension were equivalent to the antennules in Limulus, and that the antennas were 

 suppressed. 



If, on the contrary, we take Slimonia by itself, the fact of its antennas being furnished 

 with maxillated basal joints would favour the inference that they were equivalent to the 

 antenna in Limulus, and that the antennules were suppressed. 



But we so frequently find the same pair of organs in closely allied families, and even 

 genera, of living Crustacea so greatly modified as to present scarcely a character in common 

 save their corresponding position, that we may well suppose the differences of form 

 and office above described to be due, not to a difference in the organs, but to diversity of 

 habits in the genera ; suggesting the more actively predacious character of Pterygotus as 



