The Structure of Dinichthys 



163 



Text-figure 45. 

 The posterior part of the median- 

 dorsal plate seen from behind. 



im, impression on thickened part of longi- 

 tudinal keel; t\, posterior thickened part of 

 longitudinal keel. 



basis, narrower in the middle and again broader at the 

 point. Here is developed a deep sharply defined, semi- 

 circular impression (Text-figure 45 im). The relative 

 form and si2;e of this keel, as also the form and size of 

 the whole plate, vary in different species of Dinichthys 

 but all the characteristic elements are always present. 



The significance of this keel, well known in many 

 other Arthrodira, has been explained in various ways 

 by authors. Pander (1857) only ascertained that it was 

 a consolidated ridge. Woodward (1891.1) believed that 

 it was used "evidently for connection with neural arches 

 of the endoskeletal axis." Eastman (1906.2) thought 

 that "the function of the posterior process was assumed 

 to be in relation with swimming, and with its gradual 

 development, increased locomotive facilities were ac- 

 quired.'' Jaekel in 1902 noted that this process comes in 

 contact with the neural arches, but in 1907 he described 

 it as an attachment place for the muscles which move 

 the head in relation to the body carapace. This opinion, 

 which we also find in Adams (1919) and Stetson (1930) and which the present writer ex- 

 pressed (in 1929.1) must be acknowledged as absolutely correct. In a later section we shall 

 discuss the function of the median keel in more detail. 



Antero-Dorso-Lateral. — This plate (Text-figures 46, 47, 48 and 55 ADL) is large 

 and slightly curved. The condyle, serving as an attachment between body and head, is 

 placed nearly in the middle of its front margin (Text-figures 46, 47, and 48 ^d). On the 

 outside two large impressions, the overlapping margins, are clearly developed. The first, 

 on the upper part of the plate (Text-figures 46 and 47 MD), was covered by the fore 

 angle of MD; the other is on the lower part of the plate. It is especially deep, and many 

 sharply developed grooves stripe its surface (Text-figures 46 and 47 AL). This part was 

 overlapped by AL, with which it was strongly connected. 



The median part of the plate, between two overlapping margins, is quite plane. A 

 sensory canal runs from the condyle nearly horizontally to the hind margin of the plate 

 (Text-figure 46 ). In some specimens the present writer has found two canals running 

 parallel to one another across the plate. One specimen (in the Buffalo Museum) had 

 developed three parallel canals (Text-figure 55). It must be pointed out that these secon- 

 dary canals have nothing to do with the canal which in some forms (Coccosteus) runs 

 obliquely downward from the condyle (Text-figure 10 XIII). They must be regarded 

 only as individual variations of canal XII. Their number can change even on the right 

 and left plate in the same individual. Thus, in the previously mentioned specimen in 

 Buffalo, the triple canal is developed only on the right plate, on the left it is double. 



On the inside we must first note the strongly developed condyle basis (Text-figure 



