OP THE SPECIES. 55 



Second Family— CYTHERINID.E. 



These animals were contained in bivalve, bean-shaped shells, which are the only parts 

 preserved. They are more or less pointed towards the external wider margin ; at the 

 straight or dorsal margin they are rather thickened. They vary in size from K" to ^", or 

 even (though rarely) to 1". 



The only genus, C}jth.enna, which belongs to this family requires as yet a more accurate 

 study, as there seem to be several species which have hitherto been confounded with one 

 another. The specimens from the mountain limestone are probably, however, the only true 

 representatives of the geaus, and the species referred to Cyjiru, and found in the wealden, 

 cretaceous, and tertiary formations, probably belong to a different and peculiar group. 

 For the present we may consider as belonging to it — 



1. C. haltica ; HisiNGER, Leth. siiecica, 10, Table I, Fig. 2, and Table XXX, Fig. 1. 



2. C. pkaseolus ; Hisinger, ibid. Table I, Fig. 1, and the Antechdng i P/ii/s. och. Geot/n. 



of the same author, V, Table 8, Fig. 3. Klöden's Verst. d. Mark Brandenhury , 102, 

 Table I, Fig. 10-11. 



Third Family— TRILOBIT^. 



The body covered by an affixed shell, which consists of as many rings as there are 

 joints of the thorax (?); the head and the abdomen each similarly included in a large united 

 shield, which, like the rings of the shell of the body, possesses a broad border that freely 

 projects at the sides. The large eyes are situated in the lateral portions of the cephalic 

 shield, remote from the true head. A peculiar suture passes through the cephalic shield, and 

 divides it into two, three, or four special parts. The numerous members of this family, which 

 have been already particularly described in the first chapter, admit of a further division 

 into natural genera, the most suitable classification of which may perhaps be the following : 



1. Trilohites incopahle of rolUng themselves ujj. The Trilobites of this division appear to 

 be decidedly of more ancient geological date than those of the following divisions, and are 

 principally found in the lowest Silurian rocks, but are there accompanied by some species 

 of the other group. They are easily recognized by the structure of the lateral lobes of the 

 body, which is such that throughout its length each preserves the same breadth, and never 

 diminishes on the upper and external side towards the margin, a peculiarity observable in 

 the members of the second group. The lateral lobes of these Trilobites, therefore, represent 

 narrow parallel courses, which are usually divided in a diagonal direction by a transverse 

 fuiTow. On a more accurate examination, however, we find that two different types 

 predominate in their configuration, which present new differences, the details of which we 

 need not dwell upon in this general description. The species of this first principal group 

 are further distinguished by the smallness of their eyes, which are depressed, and have a 

 more elongated form than the eyes of other Trilobites. Only the inner surface of the shell 

 is known, the upper surface being absent in almost all the specimens hitherto found, ex- 

 cepting those belonging to the genera OdontopJeura, Brontes, and Harpes. This is especially 

 the case in those found in grauwacke, clayslate, and alum-slate. As already mentioned, this 

 group is divided into two sections, according to the characters of the lateral lobes. 



