222 BULLETIN OF THE 



PLATE IV. 



Fig. 1. The head of Ceraurus, with the dorsal shell broken away over the 

 cephalic cavity, showing a cast of the interior and the enlarged 

 opening of the intestinal canal. 



Fig. 2. A transverse section of Fig. 1 across the third thoracic segment. 

 The section of visceral cavity and intestinal canal are the only 

 traces of parts other than tlie dorsal shell. The light spot in the 

 centre of each dark spot represents the light shining through from 

 the front. The division of the intestinal canal into two parts 

 is undoubtedly of accidental occurrence. 



Fig. 3. A detached thoracic branchia frona the interior of an enrolled Caly- 

 mene. 



Fig. 4. Same from a Ceraurus, showing the manner in which the upper 

 portion of the spiral is strengthened by an attachment to an arm- 

 like support. 



Fig. 7. The original specimen from which the interior cast is sketched was 

 so badly broken that the outlines of the dorsal shell were taken 

 from another specimen, and the break in the shell and interior 

 from the original. The figure explains itself, as the cast of the 

 basal joints of the legs and the openings leading into the legs is 

 seen, and also the divisions of the ventral membrane leading to 

 the central ridge. 



Fig. 5. The interior surface of the dorsal shell of Ceraurus, showing the 

 hypostoma in position and the very irregular surface of the thorax. 

 The bifurcation of one of the cephalic spines has not been noticed 

 in any other Trilobite, bearing spines, that has passed under my ob- 

 servation. It is analogous to the bifurcation of the terminal spine 

 of Limulus. The two small, oval, dark spots on the inside of the 

 first segment of the pygidium are always present in well-preserved 

 specimens. They indicate a depression, .and it may be an opening 

 through the shell, but the exterior shows no trace of them.* 



Fig. 6. Median longitudinal section of Ceraurus. The dorsal shell and hypo- 

 stoma are alone preserved in the section. The line of the in- 

 testinal canal, i, and the ventral membrane, v m, are drawn to 

 show their position as far as has been determined by the sections. 



Fig. 9. Third and fourth thoracic .segments of Cyamus Scammoni Dall. This 

 is introduced to show the peculiar form of the branchiae, h, b, for 

 comparison with the spiral branchitB of the Trilobite. Dr. Dall 

 describes the branchia3 as loUows : " The third and fourth seg- 

 ments each have a branchia attached on each side. This, near 

 the base, divides into two cylindrical filaments spirally coiled 



• For a detailed description of this figure see Annals New York Lyceum Nat. 

 Hist., XL pp. 159-169, 1875. 



