1 I 



TRILOBITES. 



TRILOBITES. 



1100 



to found on the lower surface of the head on each side of the mouth, 

 aa in the Pkyllopoda ; but he is compelled to admit that no specimen 

 ha* yet been discovered which showed the slightest trace of them, 

 remarking that there would be nothing astonishing if the antonme, 

 become already rudimentary and reduced to two in Apia, should in 

 the Trilobite be entirely wanting. 



The same author calls attention to the sutural lineation, termed by 

 him the jugal line, which exists on each side of the upper surface of 

 the bead, and is more or less flexuous, springing from the posterior 

 border, passing along the eyes, and reaching the frontal edge. 



The lower surface of the head is occupied in front by a flat surface, 

 sufficiently resembling, in the opinion of M. Milne- Edwards, that which 

 exists io Apvt and I.imuliu, but which is divided by prolongations of 

 the jugal sutures, into two or three pieces, according as these lines 

 unite on the median line of the front before they are recurved down- 

 wards and backwards, or at least remain separated for a more or less 

 considerable space. Behind this region traces of the buccal apparatus 

 have been detected, though but little appears to be known relative to 

 its conformation. Dekay, C. Stokes, and Sars, have found a plate or 

 lamina bifurcated posteriorly, which, in the opinion of M. Milne- 

 Kdwards would seem to constitute a labrum, or epistomian piece 

 analogous to that which gives insertion to the labrum in certain 



' five joints ; and the succeeding rings seem to have borne similar leaf- 

 like feet of an equal size. The abdomen consisted of throe or six 

 rings, and was terminated by a pair of rudder fins. There is only 

 one genus belonging to this family, 



Under inrhcc of the anterior portion of the shield of Ataj>!nit plalyetphalut, 

 from Lake Huron. (Stoke*. See ' Geol. Trn.,' N.S., vol. i., pi. 27.) 



Dr. Buckland, in his ' Bridgewator Treatise,' speaks of the great 

 extent to which Trilobites arc distributed over the face of the globe, 

 and their numerical abundance in the places where they have been 

 discovered, as remarkable features in their history. He notices their 

 occurrence at the most distant points both of the northern and south- 

 ern hemisphere. " They have been found," says Dr. Buckland in con- 

 tinuation, " all over Northern Europe, and in numerous localities in 

 North America; in the southern hemisphere they occur in the Andes 

 and at the Cape of Good Hope. No Trilobites have yet been found 

 in any strata more recent than the carboniferous series; and no 

 other Crustaceans, except three forms which ore also Kntomostracous 

 [SroxDTUDX], have been noticed in strata coeval with any of those 

 that contain the remains of Trilobites : so that during the long periods 

 that intervened between the deposition of the earliest fossiliferous 

 strata and the termination of the coal-formation, the Trilobites appear 

 to have been the chief representatives of a class which was largely 

 multiplied into other orders and families after these earliest forms of 

 Marine Crustaceans became extinct." 



The following is an outline of the arrangement of the species given 

 in Uurmcistcr's ' Organisation of Trilobites,' translated for the Ray 

 Society, and edited by Professor Bell and the late Professor Edward 

 Forbes: 



The Poltco&e are CruiUceous Arlkulato, belonging to the second 

 order of the class Cnutacea (divided into Aitracodermata, Atpidoi- 

 traca, and i'ntomottraca), characterised by the possession of two 

 large compound eyes, by the absence of simple secondary eyes, and 

 by having short undeveloped feelers, and soft leaf-formed feet, (waring 

 gills. By these characters they are immediately related to the Phyllo- 

 poda, and might perhaps even be united with the latter in one tribe. 

 Their principal difference would then consist in the absence of the 

 constant numerical proportion of eleven rings of the thorax, common 

 to all the I'hyllopwlo, which must be expressed by the formula of 

 4x3 1. Instead of this the Palirada exhibit fluctuating numerical 

 proportions in the part* of the thorax, respecting the reduction of 

 which to a constant formula nothing certain can be said, because we 

 neither know the number of the accessory parts of the mouth, nor 

 the position of the sexual openings. These animals underwent a pro- 

 gressive metamorphosis; they moved by swimming probably with 

 their backs downwards, and they inhabited the oci an, living chiefiy 

 in shallow water. The whole group is divided into three families, 

 which are characterised according to the nature of the shelly covering. 

 Family 1. furyplerida. 



In these there is no shell The head, whose position is very distinct, 

 bears two pairs of setaceous feelers, and one pair of accessory parts of 

 the mouth. There are probably nine ( ? ) rings in the thorax, the first 

 of which bean a pair of very large rudder-shaped feet, furnished with 



Family 2. 



These animals were contained in bivalve bean-shaped shells, which 

 ore 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 4" to j", or even, though 

 rarely, 1 ". The only genus is Cythtrina. 



Family 3. TrilMta. 



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 described admit of a further division into natural genera, the 

 most suitable classification of which may perhaps be the following : 



I. Trilobites incapable of rolling themselves up. 

 Group 1. The lateral lobes of the rings of the body are situated in 

 the same plane, and do not curve or bend downwards, but terminate 

 towards the posterior part in a more or less prominent point, some- 

 times very long, which forms a somewhat obtuse angle in its principal 

 direction with the direction of the lobe. 



.1. Trilobites with a simple large caudal shield (not much smaller 

 than the cephalic shield), the axis of which is many-jointed, and 

 equals, or even exceeds, the length of the body. Oyygiida. To this 

 group belong two genera: 



1. Trinucleut, with six rings. 



2- Ogygia, with eight rings. 



Tritiucltlts Lloytiii, 



S. Trilobites having a simple though tolerably large caudal shield, 

 the axis of which consists of but a few joints, and is always shorter 

 than the body. The genera belonging to this division may be divided , 

 according to the number of body-rings, into S-jointed and 10-jointed. 



3. Odontojilcura. 



4. Arget. 



5. Brontew. 



C. Trilobites having a simple but very small caudal shield, the 

 axis of which is many-jointed, but which is always shorter thun 

 the body. 



6. Paradoxidet. 



7. Oknut. 



Paraiotiia Ttttini. 



Group 2. The lateral lobes of the body-rings not horizontally 

 extended in their whole length, but turned downwards from the centre, 

 and not terminating in a point, but with an arched and rounded 

 extremity. Furrowed on the surface along their wholo length. 



8. Cenocephalia. 



9. Kllipiocephalia. 

 10. Jiarpet. 



