EXPLANATION OF THE PLATES. 



PLATE XXV. 



Fossil Orthocerae from Thessalon Island, Lake Huron : page 198. 



The three figures in this plate (drawn of the natural size) belong to the same 

 species, and exhibit a remarkable structure in the siphuncle, which is large in 

 proportion to the size of the shell, and is dilated between each of the septa. It 

 contains within it a tube running throughout its whole length, diminishing 

 towards the lower end of the shell, and having radii in verticillations connected 

 with the sides between each enlarged portion of the siphuncle. 



Fig. 1. shows the external form of the lower part of the shell ; and in the upper 

 portion, the siphuncle laid open, and the verticillated radii distinctly separate 

 from each other. 



Fig. 2. represents the siphuncle laid open, but the radii have not been preserved in 

 this specimen. The interior tube diminishes irregularly towards the smaller 

 end, and seems to indicate that this part was formed of a substance which could 

 be dilated or contracted. This figure is drawn in an inverted position. 



Fig. 3. represents the form and size of the shell and siphuncle, with its internal 

 tube. The radii are wanting in this specimen. 



PLATE XXVI. 



Orthocerae from Lake Huron : page 196. 



The figures in this plate exhibit varieties in the external form of the shells, 

 and of the form and proportionate size of the siphuncle. There is also great 

 difference in the size of the chambers as shown by the different distances between 

 the septa. These differences are sometimes considerable in the same specimen, 

 as in figs. 3 & 8. 



PLATE XXVII. 



Four views of the natural size, of a new species of Trilobite, named by Mr. Stokes, 

 Asaphus platycephalus, from St. Joseph's Island, Lake Huron : p. 199. 208. 



Fig. I. a. Upper view of the anterior part, showing the position of the eyes, and the 

 lines of the sutures ; which, as in all the species of this tribe, divide the shelly 

 crust at the eyes, and pass in a curved line near to the anterior margin ; at the 

 centre of this margin another suture joins the curved one, and passes round to 

 the under side. 



Fig. \.h. Under view of the same. From the centre of the anterior margin, the 



straight suture above mentioned passes along the middle of this under side, till 



it meets another suture, where a plate, of the peculiar form represented in the 



figure, is attached. This plate of peculiar form has been detached at its suture 



VOL. VI. 2b 



