320 CRUSTACEA. 



the two large lobes are formed by the great expansion of the sides of the 

 thorax, which resembles wings, are almost oval, and thrown behind; that 

 the two others are clusters of eggs, &c. 



Nicothoe astaci, Aud. and Edw. The only species known; it is about 

 half a line long and three lines broad, the thoracic enlargement included. It 

 is rose-coloured, paler on the oviferous sacs; the expansions yellowish. It 

 adheres closely to the branchiae of the Lobster, and penetrates deeply be- 

 tween the filaments of those organs. 



TRILOBITES. 



According to Brongniart and various other naturalists, it is in the 

 vicinity of the Limuli and other Entomostraca with numerous feet, 

 that we should place these singular fossil animals, originally con- 

 founded under the common name of Entomolithus paradoxus, and 

 now designated by that of Trilobites. By this hypothesis we have 

 to admit as a positive or at least highly probable, fact, the existence 

 of locomotive organs, although, notwithstanding the most careful 

 investigation, no vestige of them has been discovered. Presuming, 

 on the contrary, that these animals were deprived of them, 1 thought 

 that their natural position was in the neighbourhood of the Chitones, 

 or rather that they constituted the original stock of the Articulata, 

 being connected on the one hand with these latter Mollusca, and 

 on the other, with those first mentioned, and even with the Glomeres, 

 to which some Trilobites, such as the Calymenes, appear to ap- 

 proximate, as well as to the Chitones, inasmuch as by contracting 

 they could also become spherical. Be this as it may, these animals 

 appear to have been annihilated by some ancient revolution of our 

 planet. 



The Trilobites, one heteromorphous genus excepted, that of Ag- 

 nostus, have, like the Limuli, a large anterior segment in the form 

 of an almost semicircular or lunated shield, followed by from about 

 twelve to twenty-two segments, all transversal except the last, and 

 divided by two longitudinal sulci into three ranges of parts or lobes, 

 whence their name of Trilobites. Some naturalists call them 

 Entomostracites . 



They are divided by Mr Brongniart into the following genera: 

 Agnostus, Calymene, Asaphus, Ogygia and Paradoxide*. 



' 



