. The Bashford Dean memorial volume : archaic fishes. Fishes; Sharks; Fishes, Fossil. The Structure of Dinichthys 135 Museum in 1930 found, among compara' tively good material of Coccosteus decipy ens Ag., a head (No. E1257) which shows a doubtful element at the extreme angle of the head (Text'figure 19). In all proba' bility it is PM. However, it is not very surprising that PM has never been found and described in Coccosteus. It is very small, only a few millimetres long, and difficult to see. Even in Dinichthys, where PM is 5 to 6 cm. long, it has never been observed befoie. What is especial
. The Bashford Dean memorial volume : archaic fishes. Fishes; Sharks; Fishes, Fossil. The Structure of Dinichthys 135 Museum in 1930 found, among compara' tively good material of Coccosteus decipy ens Ag., a head (No. E1257) which shows a doubtful element at the extreme angle of the head (Text'figure 19). In all proba' bility it is PM. However, it is not very surprising that PM has never been found and described in Coccosteus. It is very small, only a few millimetres long, and difficult to see. Even in Dinichthys, where PM is 5 to 6 cm. long, it has never been observed befoie. What is especially remarkable is that this plate does not occur Text'figure 18. The left hind corner of the head of Dinichthys inter- medius Nwb., showing the relative positions of PM, M and £0. In the collections of the American Museum alone, it is to be found in three head roofs, in addition there are 5 or 6 isolated plates in perfect preservation. As a curiosity, it might be mentioned that one of these plates is labelled in Newberry's hand, ''New genus and ; Only in Branson's reconstruction of Dinichthys terreUi Nwb. (, .2, 1911) do we find some traces of this plate. As has been mentioned, in this reconstruction (Text' figure 6) there is shown a new sensory canal, which goes nearly across to the outward branch of the M sensory canal (Text'figure 12 V). It is enough to compare Branson's figure with mine to be sure that Branson was mistaken and has taken a suture between M and PM for a sensory canal (Text'figures 9 and 12). Central.—This plate (Text'figures 12, 13, 14 and 15 C) is the largest one of the whole roof, and is placed in the central part of the head. Along the median line it meets its corresponding plate from the other side. On the inside, C is naturally divided into two parts: one, the hind, thick; the other, the fore, thin. The posterior part is nothing other than the "connecting bridge" mentioned before. The thin part forms the bottom of
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