/. — Pleuracanthidce. 719 



the lateral plates are squeezed towards the middle, so that the orbits are more a 

 matter of inference than certainty. Outside these again, and forming the lateral 

 margin of the cranium on each side, are two plates {g and h) represented approxi- 

 mately in their natural position in figure 2 a, and in figure 2 by the same letters. 

 Both the plates have been displaced, the anterior portion of each being pressed 

 under the preceding one. The form of each, however, is clearly seen on the 

 under surface of the specimen. Anterior to the frontals, and occupying a median 

 position, is a small hexagonal jDlate (a;). To it are attached, besides the frontals, 

 the inner lateral plates (/), and in front, extending towards the snout, a pair of 

 large semi-rhomboidal plates (i, i). The posterior margins of these are joined 

 to the anterior margins of both the inner and outer lateral plates. 



The cranium thus constituted is circular in front, expanding backwards and 

 forming a wide extension in the occipital region. The width across the latter 

 is about 0"10m., and the distance from the anterior extremity of the snout to the 

 outer margin of the occipital plate is 0-07 m. 



The arrangement of the teeth is exhibited very clearly in a specimen in the 

 collection of Mr. Greorge Wild, from the Thin Bed Coal at Burnley in Lanca- 

 shire (PI. Lxviii., fig. 4). The teeth are comparatively small, their total length 

 being 0'007m., of which the basal part occupies 0"002m., and the two principal 

 cusps 0"005m. The cusps are long, slender, divergent; between the two, the 

 median cusp ascends ; it is fully half the length of the principal ones, very 

 graceful and slender. Posteriorly at the base of the two principal cones there is a 

 large and prominent circular bulb ; the base of the tooth is antero-posteriorly 

 broader than it is between the two sides. More than fifty teeth are preserved in 

 this slab, and appear to be derived from both the upper and lower jaws. Two or 

 three rows are preserved in sequence. In one row there are six teeth, and in 

 another there are five (fig. 4). These are probably from the lower jaw. The 

 opposing teeth have the cusps pointing in the opposite direction and towards those 

 of the upturned ones of the lower jaw; they are not in rows, but more or less 

 disturbed ; they are smaller than the others, and the median cusp is longer in 

 proportion to the lateral ones, otherwise the teeth possess similar characters. 

 They are of the form described by Mr. A, Smith "Woodward as Diphdus tenuis, 

 and as this is now shown to be associated with Pleuracanthus {Diplodus) gibhosus, 

 the Burnley specimens must also be included. Though in this specimen there is 

 not any appreciable variation in form, this may be due to the small portion of the 

 whole mouth which is preserved. 



The specimen (PI. lxix.) is from the Atthey collection, presented to 

 the Newcastle Museum by Lady Armstrong, and exhibits the right ramus of 

 the lower jaws {a) with a portion of the left ramus connected to it at the 

 symphysial extremity {b). The length of the jaws is 0'36 m. The greatest 



IKANS. EOr. DTJB. SOC, N.S. VOL. TV., PAET SIT. 5 IT 



