T'LATYCUINID.'E. PL.VTYCRINITES. 29 



The Abdominal plates — are irregularly hexagonal, frequently approaching to an oval 

 figure. The integument which these plates protect(>tl was capable of a slight degree of 

 extension or contraction at the will of the animal, so that when the extensor muscles 

 were exerted, the abdominal portion became very slightly protuberant and the mouth 

 prominent ; but when the muscles were relaxed the part gradually subsided, and the 

 valvate plates surrounding tlie mouth became closed, so that the oral apertiu-e was no 

 longer visible. 



The extent to whicli the stomach and mouth coidd be protruded, is shewn in PL 2. 

 fig. 3. m. The outline of the plates in the specimen from which our figure is taken is 

 not very distinctly defined, o^\ing perhaps to weathering, or not improbably to the 

 action of acids used in the process of clearing the specimen. 



The Meso-plates — are five in number, their shape polygonal, mth their lower ends 

 pointed so as to enable them to fit into the angles formed by the union of the perisomic 

 plates. The upper portions of the meso-plates are fringed with seven or eight small 

 abdominal plates, which in well preserved specimens present a very regular and beautiful 

 appearance. In the Plati/crinites, as before observed, the meso-plates avlU be foimd 

 generally larger or smaller according to the depth of the excavations in the ray bearing 

 plates. 



The Maxillary plates. — Though we possess no evidence whatever to induce the 

 belief that these animals were furnished with jaws, yet we find it convenient to use the 

 term maxillary as applied to the plates which surround and close in the mouth of those 

 species whose oral apparatus is not elongated into a tubular form. Thus the four central 

 plates, as exhibited in figure 3. /. of our second plate, have been so termed in order to 

 distinguish them from the surrounding plates, whose oflSce was clearly subordinate to 

 the maxillars. These foxu' plates may be considered as so many valves, which the 

 animal possessed the power of opening and closing at will. When they were closed the 

 true oral aperture was so protected from external injury, that nothing but extreme 

 violence could render it liable to annoyance. 



The four maxillary plates wliich surround and close in the mouth of this species, bear 

 some resemblance to a flower with four petals ; from which circumstance it has derived 

 its specific name. 



The Ray's. — We have not, unfortunately, met with any of this species to which the 

 arms are attached beyond the basal joints, and the first scries resting on them. The 

 arrangement of these joints indicate that the rays were not numerous, probably not 

 exceeding twenty. It is worthy of observation, that m all the species with mouths but 

 slightly produced, and of which specimens have been obtained, no instance has occurred 



