28 VISIBLE STRUCTURE 



of the genus Amplim in their entire extent, also by Arr/es and Odontophura. Dalman enu- 

 merates nine rings in lUcenm cettlrofiis, but one ring might perhaps have escaped the attention 

 of the observer, since they are so remarkably small in this species. With perfect certainty, 

 I only find this number in Archcgonm (De Koninck's PhiUipsia). The true Illani have ten 

 rino-s. Dalman's Cahjmene concinna possess the same number, and if on that account only 

 cannot belong to Cahjmene, nor can it indeed be referred to Amplim, among which Emmerich 

 places it. The species of Phacops have eleven rings, EUipsocephalus twelve, Cali/mene thirteen, 

 Oleims gibbosus and Conocephalus fourteen, Olenus spinulosus sixteen, and Parado.cides bohemiciis 

 has twenty rings. This seems to be the highest number of rings existing. 



SECTION XII. 



The abdominal or caudal shield {scutum canda; ^.pggidlim), which we have next to describe, 



has already been mentioned as an extension of the coverings of the real abdomen, analogous 



to the cephalic shield, and has been exhibited as consisting of two layers, of which the upper 



possesses the same quality as the rest of the upper surface of the shell, while the latter more 



tender layer is marked with lines on its open surface in a similar manner. Between both of 



these, however, a thicker layer of the substance of the body must have existed. There 



remains therefore now only to treat of the axis of the shield, the true abdomen, in which we 



may also usually observe an articulation, although the rings are never so distinctly and 



regularly disposed as in the thorax. With respect to their distinctness, three stages may 



be enumerated, which may be termed the stage of perfect distinctness, of the indication, and 



of the deficiency of rings. The genera Tmiudeus, Ogygia, Calymene, Phacops, JEonia, exhibit 



perfectly distinct rings. Very distinct rings are also to be observed in Olenus gibbosus. The 



arch of the ring itself in this case is continued to the lateral portions of the caudal shield, 



but the number of ribs is usually less by one or two than the number of rings in the" axis ; 



at least, I have counted only seven ribs in Phacops latifrons, while there are eight, or even 



nine rings in the tail, of which the last two are certainly very small, and merge into each 



other. Calgmene Blumenbackii has always five ribs at the caudal shield, but seven distinct 



articulations in the tail itself. In Ogggia Bitcliii I have counted eleven ribs at each side of 



the caudal shield, but twelve rings at the tail itself, of which the last has a long oval shape, 



and in all probability consists of several articulations. Phacops caudatus has at each side 



seven I'ibs divided by a groove, and thirteen distinct articulations, besides an oval terminal 



articulation, which may be considered as a union of several articulations ; Phacops Hausmanni, 



finally, has most of all, namely, nineteen to twenty-one in the axis, and thirteen to fifteen 



grooved ribs at each side. The terminal articulation of all the Trilobites is of a similar 



nature as described, and therefore probably only not articulated at the upper part, because 



the thick crust prevents the ring from becoming distinctly visible. In Moni a {Gerastos Goldf.) 



I have counted seven very distinct articulations in the axis, but I have not perceived any 



ribs at the side of the caudal shield ; in Olenus gibbosus, on the other hand, six rings in 



the axis, and five on the shield, may with certainty be recognized. 



