272 S. W. Ford—Embryonic Forms of Trilobites. 



is, for the most part, a cast in stone of the outer or lower sur- 

 face of the "doublure," which is shown to be ornamented in 

 much the same way as the upper surface of the shield. Along 

 the outer edge there is a portion of the doublure itself remain- 

 ing. 



HI. The Hypostoma and Epistoma. 



Of the hypostoma of this species I have several specimens, one 

 of which, a very perfect example, is here figured twice the 

 natural size (fig. 6). Its form and structure both show it 

 to be very closely related to the hypostoma of Paradoxides. 

 The surface is smooth. The central body is exceedingly convex. 

 In his description of the type of the genus, 0. Thompsoni (13th 

 Eegents' Eep. and elsewhere), Professor Hall says of the hypos- 

 toma that it is " broadly ovate, little longer than wide." As 

 this description was too imperfect to be made available in com- 

 parison, and was unaccompanied by a figure, we were thus left in 

 doubt regarding the testimony of" this, one of the most import- 

 ant parts of trilobites that can be employed in the identifica- 

 tion or differentiation of genera. In the form of the hypos- 

 toma, the shape of the glabella, and in its fades, this genus is 

 evidently closely related to Paradoxides, more closely I believe 

 than to Olenus, as was 1 by most paleontolo- 



gists who investigated it The glabella in all the species is 

 widest at the frontal lobe. In the case of 0. asaphoides we see 

 that this is a feature that is early marked out in the structure. 



In a small number of species of trilobites described and 

 figured by Barrande, there is shown to exist a second plate 

 (luite similar in shape to the hypostoma, but placed in the 

 interior of the cephalic shield. This he has called the "epis- 

 toma. We have not hitherto found this organ in any Ameri- 

 can trilobite^ About three years ago I obtained an enormous 

 glabella of 0. asaphoides, beneath which there appeared to be 

 at one point indications of the existence of a plate of different 

 character from anything I had previously seen. The specimen 

 remained in my hands in this state for a long time, but I 

 finally succeeded in developing the body referred to, which is 

 the remarkable form represented (nat. size) in fig. 7 of the plate. 

 It shows a central or sub-central elevated area with two deep 

 oblong parallel furrows; an elevated left-hand border, and 

 between these a wide concavity. At the upper left-hand por- 

 tion is the cast of a stout spine. Just to the right of this a 

 portion of the plate has been removed. The surface presents a 

 smooth polished appearance. This plate I believe to be the 

 epistoma of this species. 



The specimen whence this plate was obtained consisted as 

 originally discovered of two parts, one showing the inner surface 

 of the crust of the glabella, and the other its impression in 



