Crustacea. | LOWER PALAXOZOIC ARTICULATA. 133 
TRACHYDERMA SQUAMOSA (Phil/.) 
Ref—Phill. Mem. Geol. Surv. Vol. II. t. 4. f. 3. 
A very obscure fragment, apparently of this species, has occurred in the Upper Ludlow rock of 
Benson Knot, Kendal, Westmoreland. 
TRACHYDERMA ? L&VIS (M/°Coy). Pl. 1. D. fig. 10. 
Ref —M°Coy, Ann. Nat. Hist. 2nd Series, Vol. VII. 
Sp. Ch.—Tube slightly curved, thin, coriaceous, slightly tapering, subcompressed, slightly more than 
one line in diameter at the broad end, and slightly less than one line at the imperfect smaller end of a 
specimen one inch seven lines long; surface nearly smooth. 
The specimen is a brown, tough, flexible tube, irregularly and gently compressed (parallel to the plane 
of stratification of the rock) assuming an oval section; and being filled with the bright-coloured matrix 
shews clearly the thinness of the tube, which, from the same cause, has a few irregular indentations of 
the surface, which otherwise seems smooth. 
Position and Locality —In the very fine beds of Caradoc sandstone of Acton Scott, Church Stretton, 
Shropshire. 
Explanation of Figures —PI\. 1. D. fig. 10. Specimen from the fine Caradoc sandstone of Acton Scott, 
natural size and section. 
2nd Class. CRUSTACEA. 
The animals of this class have a hard-jointed integument, composed of a thick, internal, spongy chorium 
or vascular cutis, a coloured pigmental layer, and a cuticular secreted external layer; these three layers 
are at first all equally flexible and continuous ; subsequently transverse wrinkles appear, which gradually become 
segments by the cuticle acquiring calcareous matter, principally carbonate of lime, with a little phosphate 
of lime and magnesia, (and, according to some, a little ‘chitine,’ as in insects) ; these solidified bands become 
segments by separating posteriorly from the lower layer or chorium, which remains flexible, and permits 
the various motions. Each segment is believed (for they can seldom be all demonstrated) to consist of 
six pieces, two tergal pieces above (separated by a median longitudinal suture), two similar sternal pieces 
below, an epimerian piece on the upper half of each side, and an episternal piece forming the lower half 
of each side. In the lower crustacea the segments are very numerous, distinct, and nearly alike, but they 
gradually coalesce in the higher types, coinciding with the condensation of the nervous centres. No segment 
ever bears more than one pair of appendages, a fact which is used to demonstrate the true number of the 
segments which may be anchylosed into the apparently undivided head or thorax of many groups. In the 
majority of crustacea the first seven joints belong to the head, and bear the organs of sense and parts 
of the mouth, the next seven to the thorax (according to M. Edwards), bearing the organs of locomotion, 
the last thoracic joint being always defined by the male sexual openings, and the last seven to the abdomen, 
containing the principal viscera, and having the anus in the last joint. The first ring bears the eyes when 
they are present, the second and third rings bear the two pair of antennz, which are absent only in the 
lowest types, the fourth bears the mandibles, the fifth and sixth the jaws, the appendages of the succeeding 
rings varying in shape and use according to the group.—Digestion: the complex mouth is always on the 
under side of the head, composed of the Jabrwm or upper lip, a labiwa or under lip, jaw-feet, mandibles, 
maxille, palpi, &e., which it is unnecessary to describe, followed by a short vertical cesophagus leading 
to a large globular stomach directly over the mouth (often containing sharp tubercles for triturating the 
