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Show 1002 DR. J. W. GREGORY ON A NEW [Dec. 15, close union of the plates having obscured the sutures, and to irregularity in the normal symmetry. The small size of the specimen at once raises the question as to whether it is mature or is only a form so young that it is useless to found a genus upon it. If the specimen were the only echinid in the bed from which it came, or had been associated with echinids of normal size, it would probably have been impossible to give a satisfactory reply to this objection. But Lysechinus belongs to an echinid fauna all the members of which are minute. Tlarechinus is smaller, while the species of Cidaris, Hypodladema, and Salenla are of about the same size. The specimens of the last three genera have the characters of maturity, in spite of their minuteness, and thus w e cannot take the small size of Lysechinus as a proof that it is a larval form. That it is not a pathological variation cannot be so definitely disproved. This idea seems supported by tbe fact that the radial symmetry of the specimen is not perfect. One of the inter-ambulacra is more prominent than the rest, but this malformation is as likely to be a post-mortem accident during fossilization as an ante-mortem variation. But we cannot ignore Lysechinus as a mere sport until w e know some echinid which may be regarded as the form of which it is the sport. Numerous echinids are known with some striking character which may be explained by teratology ; but in such cases there is no doubt as to the species, or at least the genus, from which the sport arose. There is no known Triassic or Palaeozoic echinid which resembles Lysechinus and Tlarechinus, and from which either genus can be conceived as having originated by a single variation. Several specimens of Tlarechinus are known, and they all agree in structure, so that that genus is not teratological; and until we know of some echinid from which Lysechinus could have sprung we cannot adopt the easy course of dismissing it as an abortion. The greatest difficulty presented by the specimen is due to the close union of the plate, whereby the recognition of the sutures is difficult. By the aid of Loven's fluid I believe that I can see sutures which show that each interambulacrum consists of nine plates, arranged as follows :- 1. Adjoining the genital plate are three quadrangular plates. 2. Three quadrangular plates, each bearing a tubercle. 3. T w o angular plates, each bearing a tubercle. 4. One peristomal plate. This arrangement is not altogether free from doubt, for it is difficult to discriminate between cracks and sutures, and they cannot be detected in all the areas l. 1 The sutures could probably be exposed by the application of weak acid but this method is not invariably successful, and so long as the specimen ie unique it is not advisable to subject it to any risk. |
