364 SYNOPTIC COLLECTION. 



In the large order with Triarthrus and Lichas, is found 

 Isotelus gigas Dekay (No. 893). This genus exhibits the 

 three regions of the body very distinctly. The cephalon 

 and caudal shield are smooth and unornamented, showing 

 only slight evidence of segmentation. The eight thoracic 

 segments, on the other hand, are distinct. The central 

 axis is unusually broad in this genus and the glabella is 

 not lobed. Close to the glabella, on either side, the eyes 

 stand out prominently (No. 893). These trilobites had 

 the habit of doubling upon themselves (No. 894), prob- 

 ably for safety. If this specimen were turned over, the 

 posterior part of the dorsal side would be seen as repre- 

 sented by the figure (PL 895). When doubled up in this 

 way the ventral side of the animal is completely hidden. 



In the most specialized order, as given by Beecher, we 

 have Calymene (No. 896) and Dalmanites (No. 897). 



The body in Calymene is more or less ornamented. 

 The thoracic region is the longest, consisting of thirteen 

 segments, while the caudal shield is tapering and bends 

 downward at nearly right angles to the body. The gla- 

 bella is deeply grooved (No. 896) and its lobes are some- 

 times mistaken for eyes. The latter organs are on the 

 free cheeks and are comparatively small. 



The body of Dalmanites (No. 897, D. limulurus Hall) 

 extends backward in a long spine (not shown in the 

 specimen) . The dorsal shield is also carried back on 

 each side as a sharp spine. The eyes in this genus are 

 generally large and are always faceted. The free cheeks 

 on which they are borne unite in front, making a com- 

 plete segment which Beecher regards as the ocular seg- 

 ment. 



Merostomata. The horseshoe or king crab, Limulus 

 polyphemus Latr. (No. 898; PI. 899; Nos. 900-903), 

 is the only representative of the Merostomata that is 

 living at the present time. In its development it passes 

 threugh a trilobite stage. This is seen in No. 898 and 

 in PI. 899, figs. 1-3. The dorsal view of the embryo just 



