The Structure of Dinichthys 



127 



DESCRIPTION OF THE ARMOR OF DmiCHrHTS 



In this section are given detailed descriptions and drawings not only of new or hither' 

 to imperfectly known plates of Dinichthys, but also of all the well known ones. 



Remarkably enough, after Newberry, in Claypole only do we find more thorough 

 descriptions of the single plates of the head of Dinichthys. The other authors have been 

 satisfied with schematic drawings and partial reconstructions on a small scale, and very 

 seldom do we find detailed pictures of plates of the carapace of Dinichthys. Nevertheless, 

 Dinichthys is an especially favorable object for thorough description. It is one of the 

 largest of the known Arthrodires and is often very well preserved. It is not too highly 

 specialized in any one direction and shows, therefore, all the more typical characters of the 

 Arthrodira. The single plates of the carapace can be worked out from the matrix and 

 observed from all sides. For these reasons, Dinichthys is much more suitable to take as a 

 type for the Arthrodira than is Coccosteus decipiens Ag., which was for many years the 

 best known form. 



In the following pages I have tried to give a detailed description of Dinichthys, 

 with as many drawings and photographs as possible, realising that drawings are the most 

 perfect descriptions. This work is based chiefly on the fragments of Dinichthys inter- 

 medius Nwb. which are best represented in the collection of the American Museum of 

 Natural History, but all the other specimens of Dinichthys are also considered. It is my 

 hope that this paper may be of help in studying other less well preserved Arthrodira, 

 and may perhaps contribute to a better understanding of this remarkable group. 



THE HEAD SHIELD 

 As we known, the head shield of Dinichthys is composed of not less than 37 single 

 plates. These plates cover nearly the whole head like a perfect armor, leaving only the 

 openings for the eyes, nostrils and mouth. The only region unprotected by plates is the 

 under part, between the lower jaws. The whole head can be divided very naturally into 

 two parts: the central part, or roof of the head; and the side parts, or the plates around 

 the mouth. 



THE HEAD ROOF 



The single plates of the head roof ^; as we shall call this part, are for the most part 

 strongly connected with one another and form a compact, well limited shield. This is 

 the case in all Arthrodira, and as a result this head roof has often been described for the 

 whole head (e.g. in Acanthaspis, Phlyctaenaspis). The 15 plates which compose the head 

 roof (Text'figures 12 and 13, and Plates I, II) are connected with one another by more or 

 less complicated sutures, all of which must be characterised as squamosal sutures. The 

 margins of all the plates overlap each other, but the limits between the single plates may 

 be very unlevel, with large and extensive indentations fitting into one another. Further' 

 more, the margin of some plates may show spines, long crests, etc., which fit into cor' 

 responding excavations in the margins of the neighboring plates As a result, the plates in 



* In the following description, the letters in parentheses refer to Text-figures 12, 13 and 14 unless otherwise indicated. 



