Entobia retiformis (Stephenson, 1952)

The species is based mainly on the casts of borings, whjch appear as irregular intercommunicating networks, chains, and clusters on the surface of the molds of mollusks left in ferruginous sandstone when the shell substance of the host was dissolved away. The young sponges entered the shell through numerous circular openings of their own making, which range !n diameter from 0.1 mm or less to about 0.5 mm. Once inside the shell the young sponges bored laterally, the individuals often meeting and fusing to form colonial assemblages. Th,e borings are extremely irregular, ranging from small to relatively large tubes, swelling at intervals to still larger irregular ovate- or jug-shaped cavities. The pattern may be that of an open lacework of labyrinthine passages or a dense mesh of tubes and turnipshaped intercommunicating cavities. The surface of the east is finely stippled, a feature which appears to be characteristic of the genus. In general the patterns formed by the casts are small and fine, the di~meters of the largest tubes and swellings rarely exceeding 1 mm except at the meeting places of two or more of the larger cavities. The growth habit of this species is similar to that of Oliona rrnicrotuberum Stephenson ( 1941, pp. 54--56), but the pattern formed by the borings is consistently very much smaller and finer, this fact, together with its lower stratigraphic position, indicating with reasonable certainty that it is specifically distinct.