198 BULLETIN OF THE 
THE STRUCTURE OF THE TRILOBITE. 
The Dorsal Shell. — The character and structure of the dorsal shell, 
and also of the hypostoma, have been so fully and thoroughly discussed 
and beautifully illustrated by M. Barrande,* that itis not necessary 
to review them here. One illustrationis given of the interior or ventral 
surface of the dorsal shell of Ceraurus pleurexanthemus, as it has not 
been illustrated in any general work on the Trilobite, and it also has 
the added interest of having afforded more sections showing the presence 
of the remains of the animal inhabiting the dorsal shell than any other 
species. .À careful inspection of Plate IV. fig. 5, will show why it is 
that in the sections, which are cut across at all angles to either axis, 
the outlines of the dorsal shell have such a variety and variation of 
form as shown on Plate III. figs. 4, 5 and 6, and Plate II. figs. 3, 6, 
and 8.f 
A longitudinal section through the median lobe, Plate IV. fig. 6, 
gives an outline of the dorsal shell, the hypostoma, the cephalic cavity 
(c.), the restored outline of the ventral membrane (v. m.), and the line 
of the intestinal canal (¢.). 
The species Calymene senaria t is a well-known form, and, with the 
closely allied form Calymene Blumenbachii, has been illustrated, as 
far as its dorsal shell and hypostoma are concerned, by various authors, 
as also Asaphus platycephalus, one section of which is illustrated on 
Plate II. fig. 9. 
The following arrangement will be observed in describing the re- 
mains of the body and appendages exposed beneath the dorsal shell:— 
1. The Ventral Membrane. 
2. The Intestinal Canal. 
3. The Appendages beneath the Head. 
4. 'The Appendages of the Thorax and Pygidium. 
5. The Respiratory Apparatus. 
The details of each section, etc., used in illustration, are given at the 
end of the paper, where the reader is referred for a more detailed 
* Sys. Sil. Boh., I. Trilobites, 1852. 
+A detailed description of the interior as illustrated on Plate IV. will be found in 
the Annais of the New York Lyceum of Natural History, XI. pp. 159-169, 1875. 
t Hereafter this species will be spoken of as Calymene, and Ceraurus plewrexanthe- 
mus as Cerawrus, as they are the only species of either genus used in cutting sections, 
and it is desirable to avoid repeating the specific name. 
