Article: The anatomical structure and systematic position of Pentablastus (Blastoidea) from the Carboniferous of Spain
K. A. Joysey and A. Breimer
Pentablastus supracarbonicus Sieverts-Doreck was described originally from only two specimens. The collection of over twenty additional specimens has now made possible a more detailed description, including an account of the internal structure. Optical discontinuities in the calcite have been used to detect boundaries between the component plates, and serial sections have been prepared by the new annular sawing technique at 0-35 mm. intervals. These methods have revealed several unexpected features, including an optical discontinuity between the body and the limbs of each radial plate. The posterior deltoid is divided, and all the deltoids have a concealed vertical limb lying internally to the radials. There are basal pits on the floor of the thecal cavity. Although the under-lancet plate is a distinct structural unit, it is divided by an optical discontinuity along the mid-line of the ambulacrum. Each half of the under-lancet plate is continuous adorally with a hydrospire-plate, so providing direct evidence in favour of a compromise solution to the long standing controversy on the nature of the under-lancet plate in blastoids.While making comparisons with other forms it has been found that the posterior deltoid of Acentrotremites is divided, and that the subradial plate of this genus is merely a concealed internal limb of the deltoid. Pentablastus and Orophocrinus show a close similarity in the form of the under-lancet plate, and in the relationship between the hydrospire-folds and the hydrospire-clefts. A distinction is made between hydrospire-slits and hydrospire-clefts, the latter being present in Pentablastus and some species of Orophocrinus, including the type species. It is suggested that those species of Orophocrinus which do not possess hydrospire-clefts should be removed from this genus. Orophocrinus is itself removed from the Phaenoschismidae and placed in the same family as Pentablastus, for which family the name Orophocrinidae Jaekel has priority over Pentablastidae Sieverts-Doreck. Amended diagnoses are given.