180 



SILURIAN TRILOBITES. 



whence a raised line runs to the margin, a common cu-cumstance where the axis is 

 abbreviated. 



" Length of head and thorax taken together three inches ; length of the supposed tail 

 (of a younger individual) five lines. 



" Locality. — Llandeilo Slates of Abereiddy Bay, Pembrokeshire ; in my collection." 

 —H. Wyatt-Edgell. 



We may now proceed with the most abnormal of all the Asaphidce — the group of 

 Illanus, so very characteristic of Lower Silurian — the " Faune Seconde " of Barrande. It 

 is true that in one of its subgenera it rises into the Upper Silurian ; but the mass of the 

 species, and especially the abundance of individuals of this genus, mark Llandeilo and 

 Caradoc rocks (Lower and Middle Bala of Sedgwick) most efiFectually. [Lower, Middle, 

 and Upper Bala rocks of Sedgwick are equivalent to Llandeilo, Caradoc, and Llandovery.] 



Ill^nus, Dolman, 1826. 



Of all the higher Trilobites, this is the most abnormal in shape ; — the segments of the 

 large head, and of the equally bulky tail, being so condensed and obliterated as to leave those 

 portions like the rudimentary extremities of an Agnostus ; while the structure of the 

 pleurae, the form of the head, eyes, labrura, and rostral shield, — the thick sculptured crust, 

 and the perfect mobility of the thorax-rings, all mark the genus as one of high rank among 

 the Asaphidce, and as nearly allied to the Proetida, the highest of the smooth-eyed groups. 



All the species of the genus have a common facies : — the similar head and tail, un- 

 marked by lobes or annulations ; the glabella, cheeks, and margin being usually confounded 

 in one hemispheric mass ; and the axis, in most instances, scarcely indicated outside the 

 caudal shield. The creature could roll itself up into a perfect ball, and, no doubt, chiefly 

 suggested to Dr. M'Leay the analogy with Bopyrus and other Isopoda ; the resemblance 

 to the Armadillo Woodlice is not less striking. Yet the Isopoda have crustaceous feet well 

 articulated, and we have yet to learn that Trilobites had any crustaceous feet at all.^ 



^ I find that every one in our own country has overlooked a very important notice, by Dr. Volborth of 

 St. Petersburg (' Verhandl. Min. Gesellsch. Petersburg,' 1857-8, p. 168), of the discovery, by Dr. Pander, 

 of some appendages, which may be membranous feet, attached to definite parts of the outer portion of the 

 pleurae beneath. I do not know that Dr. Pander has puWished this in any definite form, but Dr. Volborth 

 has fully described and figured these points of supposed attachment for swimming feet (' Mem. Acad. Imp. 

 Pctersb.,' 1863, tom. vi, No. 2). Barrande observed them in 1855, in Ogytjia, and described them in 

 J 858. Mr. E. Billings, Palaeontologist to the Canadian Survey, has also found in Asaphus gigas a trace 

 or two of curious processes below the crust, but close to the axis. All these data have great value ; and 

 those who have abundance of specimens of Asajihts expansus, from the fine Lower Silurian mudstone of St. 

 Petersburg, will do well to cut and polish specimens, observing all traces of fragments of tlie general 

 lower membrane, as well as of distinct processes (for details see 'Mem. Acad. Imp. Petersb.,' 18C3). 



