TO THE EXISTING ARTICULATA. 49 



SECTION XXV. 



On the othei- liand, however, there arises a very natural suggestion, that the Trilobites 

 may have undergone a metamorphosis similar to that described as characterizing the 

 Phi/Uopoda. I by no means question the great probability of such a modification, but there 

 is as yet no distinct evidence in support of it, for up to the present time no one has been 

 fortunate enough to discover a specimen in the young state, offering any distinct indication 

 of gradual development or metamorphosis. My original notion, that the so called genus, 

 Battiis, was founded on the shields of the heads of young Trilobites, has since been dis- 

 proved by Dr. Beyrich, who has shown, on the other hand, the probability that these shields 

 belong to a particular genus of Trilobites, the one form of them covering the head, and the 

 other the tail, while the two were connected by an articulated body formed of two rings. 

 This genus we shall afterwards describe more in detail in its proper place. 



SECTION XXYI. 



Although, according to these observations, the develojiment of the Trilobites has not yet 

 been proved identical with that of the P/i^Ilojjodu, there can still be hardly a doubt that 

 there exists a completely analogous organization in the two groups, and a circumstance 

 which has hitherto been overlooked seems to render this analogy still more probable. 

 The three principal stages of modification of the fundamental type, which at present exist 

 in the three families of the Plii/Uopoda above described, likewise seem to be presented in the 

 case of the Trilobites. In the former group we observed that there are — (1) Naked 

 Phyllopoda {Bra/ichijjus) ; (2) Plii/Ilojjoda with a simple horizontal shield {Apus) -, and (3) 

 PhyUopjodu with a bivalve, perpendicular shield {Limuadia). Precisely the same differences 

 of structure are also found among the Crustacea that lived at the period of the Trilobites, 

 and thus the analogy of those early types with their present representatives, the Phyllopoda, 

 is confirmed in a very singular manner. 



Besides the true Trilobites, of which the broad, flat, perpendicular shield leaves no 

 doubt that we must recognize it the analogue of the simple perpendicular shelled Apu>i, we 

 find also in the Palaeozoic strata the remains of two other crustaceous animals, one of which 

 {Cytheruut) was covered with a bivalve shell, offering a perfect resemblance to that of 

 Estheria, while the other {Eurypterun) was entirely without any shelly covering, and seems to 

 have been analogous to Branchipus. Of Cytherina nothing has been preserved but the shell, 

 and, according to Hisinger's figure,* it so perfectly corresponds in structure with Estheria,^ 

 that it is scarcely necessary to say another word respecting the relations of afiinity of the 



* Lethsea Suecica. 



t See Strauss, in 'Museum Senkeubergianum,' torn, ii, p. 110, Table ^ II, aud the still better 

 figure given by Joly, in the ' Aunales des Sciences Natur.' Nouv. Sor. torn, xvii, p. äü3, PI. 7-9. 

 1842. 



7 



