260 XIPHOSTJRA CYPROLDA. 



of an epidermis, exuded from the vessels of the skin, and 

 hardened with a considerable proportion of the carbonate 

 and phosphate of lime. At certain periods this crust is 

 thrown off, to permit of the growth of the animal. 



The skeletons of this class are preserved in the rocks of all 

 epochs, and certain generic forms are special to the stages of 

 different periods, which they serve to characterise. This law 

 will be better understood after taking a rapid glance of the 

 orders. 



Order XIPHOSURA, or King Crabs. Have the body 

 covered with a large cephalo-thoracic shield ; the abdomen 

 is very small, and terminated with a long styliform tail ; the 

 jaws, or organs of the mouth, resemble those for locomotion. 

 Mastication is performed by one of the joints of the jaw-feet, 

 which has a cutting border for that purpose. They have two 

 large compound, and two small simple eyes, situated on the 

 anterior and lateral parts of the thoracic shield. 



Limulus is found in the carboniferous stage of the palaeo- 

 zoic rocks, in the Oxford stage of the oolitic, and lives in 

 our present seas. Halycina belongs to the muschelkalk, and 

 Bellenurus is limited to the palaeozoic strata. 



Order CYPROIDA. Have the body enclosed in a bivalve 

 shell, united by a hinge along the back. The animal can 



Fig. 174. Fossil Cypridcs. 



1. Cypris spinigera: the small figure shows the natural size. 



"2. Ct/pris granulosa. 3. Cypris Valdensis. 



4. Clsiy, with numerous cases of the species represented ; fig. 1, of the natural SIZP. 



