REPTILIAN AGE. JURASSIC PERIOD. 



119 



Fig. 1. 



has shown that it is against all analogy to regard them as the valves of Cirripedes,^ 

 the most generally received opinion is that they are the opercula of Ammonites, 

 Scaphites, Goixiatites, &c.- This opinion would also seem to receive considerahle 

 weight from the discovery, in few rare instances, of an Ammonite with a Trigo- 

 nellite closing and apparently exactly fitting its aperture.^ 



Before bringing forward some reasons, however, for doubting the correctness of 

 this conclusion, suggested by a remarkable Trigonelhte found in a Scaphite from the 

 Cretaceous rocks of the Upper Missouri country, the following description of this 

 provisional genus is necessary for a clear understanding of the subject: — 



As usually found, these bodies consist of one or two (most frequently two) ovoid or 

 subtrigonal plates or valves, with one extremity truncated and generally wider than 

 the other, and one side nearly or quite straight; while the outer or lower surface is a 

 little convex, and the inner concave. Unless displaced or separated by some accidental 

 cause, the two pieces always occur with the straight edges joined 

 together in such a manner as to indicate that they were originally 

 held together in some way. The normal position of the two 

 pieces seems to be like that of the valves of a bivalve shell partly 

 open ; but when they are opened out and flattened by pressure, 

 as is often the case, they frequently present the bilobate appear- 

 ance seen in the annexed cut. Fig. 1. In structure, composition, 

 and thickness they present differences, in consequence of which 

 the group has been divided into the three following sections : — 



1. The Cellolosi, which are thicker than those of either of the other sections, and consist first of a thin, concentri- 



cally striated inner layer, over which there is a thick calcareous portion, composed of numerous polygonal 

 tubes arranged with their longer axes at right angles to the outer and inner surfaces of the valves. Outside 

 of this there is a thin calcareous layer, tlie smooth surface of which is usually perforated by small pores. 



2. The Imbkioati, with an inner layer like the preceding, and a middle porous stratum, the tubes of which are 



smaller and less distinctly defined. Its outer layer also differs in being a true calcareous shell, with 

 distinct plications, and a smooth punctate surface, the punctse of which are arranged in regular lines. 



3. The CoKNEi, which are said to consist of a single very thin corneous lamina, destitute of any porous or tubular 



layer. 



Our Upper Missouri specimen, already alluded to, is very thin, and seems to be 

 composed of a single lamina showing no cellular structure, and may consist only of 

 the inner layer. It occurs in the outer chamber of a Scaphites Cheyennensis, and is 

 the only organic body found in it, the surrounding space being filled with the fine 

 indurated sedimentary matter, similar to that in which the Scaphite was originally 

 enveloped. It occupies a position apparently about one-fourth of the distance back 

 from the aperture to the first septum, and lies with the two valves partly open, and 

 apparently in their natural position with relation to each other, their straight edges 

 being joined together, and deflected upwards so as to form a distinct carina, which 



* The reason offered by Mr. Darwin for rejecting the conclusion that these bodies are the valves 

 of a Cirripede is, that they are nearly always found with the straight edges of the two valves or 

 pieces in contact, so as to show that they must have been anchylosed or held together by a membrane 

 or kind of ligament along these margins, which would have been impossible if they were the valves 

 of a Cirripede, since that is the very side where the feet would have to pass out. 



' They have been found associated with Goniatites in Devonian rocks. 



s See an example published by Mr. Woodward in the Geologist, vol. Ill, 1860, p. 328. 



