Spfcies of thk Kbokitk GrRorip. 



181 



MODicTYA. Apertural region somewhat more expanded than the base ; margin 

 of aperture regular and without tufts. Surface smooth, or with traces of 

 obscure, discontinuous vertical ridges. 



Reticulum, very fine. The specimen is so preserved as to expose for 

 nearly its entire length the inner surface of the wall, this being somewhat 

 abraded about the base. At the summit a portion of the internal cast adheres, 

 so that the entire thickness of the skeleton is here retained. Toward the 

 lower part of the sponge may be seen remnants of two small, widely separated 

 vertical bundles of stout smooth rods. No other vertical rods are apparent, 

 while the horizontal rods, though minute, are so abundant and so matted 

 together as to form a felt of spicules without any division into bundles. The 

 prevalence of these quite obscures all traces of reticulation. The flesh spicules, 

 which are extremely abundant over the upper part of the specimen, are large 

 oxyhexasters with their rays variously modified; sometimes four of these 

 rays are simple, while those of the third axis are divided each into three 

 prongs ; in other cases the rays of a single axis are simple and those of the 

 other axes divided into three or four prongs. The prongs are not always of 

 the same number in a hexaster, some of the rays bearing three, some four, 

 and the prongs themselves being variously subdivided and often j)roducing 

 very complicated forms. There is some variation in size in these spicules as 

 shown in the accompanying figures. Besides these hexasters, which seem to 

 compose the greater part of the spicular mass, there are occasional fragments 

 of regular hexactins, and numerous fragments of hexactin rays showing very 

 finely echinate extremities. There are also two styles of umbrella-shaped 

 clavules, one having a sharply tapering head with broad divisions, eight in 

 number, the other considerably larger, with apparently seven highly divergent, 

 narrow and acuminate divisions. 



The specimen bears no little resemblance to a small, or young individual 

 of Phragmodictya catilliformis, though showing no evidence of basal plate or 

 frill, and it was regarded as that species until the examination of its skeletal 

 structure demonstrated the impropriety of referring it either to this or any 

 other known species of Dictyosponges. 



Dimensions. Length, 54 mm. ; basal width 20 mm. ; median width 

 15 mm.; apertural width 25 mm. 



Locality. In the calcareous shales at Crawfordsville, Indiana. 



