Ch. XXYL] 



OLD RED SAXDSTOXE. 



527 



gigas, Salt., from the Upper Ludlow Rock, attained dimensions fu.ly 

 as large, even up to 7 feet. 



Fie. 590. 



Portion of the Pterygotus anglicus, Agassiz. 



1. iliddle portion of the M Seraphim, 1 ' or hack of the head, with the scale-like sculpturing. 



2. Portion of the dilated base of one of the anterior feet, with its strong spines or teeth, 



nsed as masticating organs. 



3. The proximal portion of one of the great anterior claws. 



4. Termination of the same, with the serrated pincers. (See Agass., Poiss. Foss. du Yieux 



Gres Eouge, plate A.) . 



1 and 2 are of the natnral size ; 3 and 4 are reduced one-half. 



Fig. 591. 



Pterygotus anglicus. Ag., Forfarshire. 

 Ventral aspect. Eestored by H. Wood- 

 ward. F.G.S., from nearly perfect specimens 

 of allied species, found in the Upper Lud- 

 low Eock of Lesmahagow. 



a. Carapace, showing the large sessile 

 eyes at the anterior angles. 



o. The rnetastoma or post-oral plate 

 (serving the office of a lower lip). 



c. c. Chelate appendages (antennules). 



d. First pair of simple palpi (antennce). 



e. Second pair of simple palpi (mandi- 

 Mes). 



f. Third pair of simple palpi (first max- 

 UUb). 



g. Pair of swimming feet with their 

 broad basal joints, whose serrated edges 

 serve the office of maxilla:. 



h. Thoracic or genital plate covering the 

 reproductive organs (and probably also the 

 branchiae), composed of two broad lateral 

 ala?, and a slender median lobe, varying in 

 form according to the sex. This thoracic 

 plate covers the first two thoracic seg- 

 ments, which are indicated by figures and a 

 dotted line. 



1-6. Thoracic segments. 



7-12. Abdominal segments. 



13. Telson, or tail-plate. 



The largest crustaceans living at the present day are the Inachus 

 Kcempferi of De Haan, from Japan (a brachyurous or short-tailed 

 crab), chiefly remarkable for the extraordinary length of its limbs ; 

 the fore arm measuring 4 feet in length, and the others in proportion, 



