. The Bashford Dean memorial volume : archaic fishes. Fishes; Sharks; Fishes, Fossil. 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 c
. The Bashford Dean memorial volume : archaic fishes. Fishes; Sharks; Fishes, Fossil. 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 () believed that it was used "evidently for connection with neural arches of the endoskeletal ; Eastman () 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 ) 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
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