H, Woodward — On a New Long-eyed Trilobite, 491 



equally long eye-stalks, the Asaphus KowalewsM from the Silurian of 

 Eussia (see Plate XXI. Figs. 4 and 5).' 



But the sessile or pedunculated form of the eyes in the Crustacea 

 cannot be held as peculiarly characteristic of any one order. If this 

 were insisted upon, we must classify such forms as Squilla, My sis, 

 PJiyllosoma, belonging to the Stomapoda, with the true Decapod 

 Crustacea.^ 



It must, however, be borne in mind that the eye itself (and not the 

 peduncle, or its exact form), is the essential organ, the peduncle 

 being merely a form of stand or support for the more convenient, 

 adjustment of the lens which conveys, by the optic nerve to the brain, 

 the impressions of external objects coming within its range. 



If we turn, for a moment, to recent Crustacea we find among the 

 Crabs (Decapoda-Brachyura) the gi'eatest possible variation in the 

 development of the eye-peduncle. Thus in the Common Crab ( Cancer 

 pagurus), the Pea-Crab {Pinnotheres) , and in Ixa, Arcania, Ebalia, and 

 Philyra (among the LeucosiadcB) , the peduncle is scarcely, if at all, 

 elongated beyond the orbit, in the concavity of which it is articulated . 



In Gonoplax (one of the ''Quadrilateral" Crabs) PI. XXI. Fig. 9, 

 it is half an inch in length, and the eye is placed at its extremity. 

 In Podopthalmus vigil, one of the very active pelagic forms of Indian 

 Crabs, the eye-stalks are of prodigious length, and are furnished with 

 a second articulation near the summit, enabling the eye to be directed 

 more readily upon any special object. In Ocypoda ceratophthalma 

 (PI. XXI. Fig. 10) the eye is placed midway upon the peduncle, 

 the eye-stalk itself being developed beyond the cornea in the 

 form of a somewhat blunt spine. The same spine-like prolonga- 

 tion of the eye-peduncle beyond the eye itself is seen in Gelasimus 

 platydactylus, one of the *' Calling-crabs" common on the sea-shores 

 of China and Japan (see PL XXI. Fig. 8). This character is not 

 however, of more than specific value in either Gelasimus or Ocypoda, 

 species occurring in both genera (all but identical in other respects), 

 in which the eyes are really terminal in position. 



In all these higher forms the eye-peduncle has an articulation at 

 its base, and is protruded from or withdrawn into the orbital fossa at 

 the will of the animal by the action of the peduncular muscles. 



In the recent sessile-eyed forms no such provision exists, the 

 cornea being but slightly raised above the surface of the head-shield, 

 as indeed in most of the Trilobites. The Trilohita alone ofi'er an 

 example of a fixed eye raised upon an immoveable eye-stalk. 



Among the Isopoda there is an interesting living form, the Cerato- 

 cephalus Graijianus (White MS.)^ from Flinder's Island, Bass's 

 Straits (PL XXI. fig. 7), in which pseudo-eye-stalks (p.p.) are 

 developed, but the eyes (o.o.) are at their bases on the sides of the 



^ For the original figures and description of this remarkable species oi Asaphm, 

 see Article XI. " Zwei Neue Asaphus-arten aiis dem Silurischen Kalksteine des 

 Gouverneraents St. Petersburg," von N. Lawrow. Taf IV. and V., p. 239, in the 

 Verhandlung. der Eussisch-k. Mineral. Gcscll. zu St. Petersburg, Jahrgang, 1855-56. 



2 In Bell's British Stalk -eyed Crustacea this has actually been done, but of course 

 the Stomapoda are a perfectly distinct group, 



3 The original specimens are preserved in the British Museum. 



