MIDDLE CAMBRIAN. 3 



Jurassic, and still earlier, in the Middle and probably in the Lower Cam- 

 brian. The conclusion is that the acraspedote medusae were mainly differ- 

 entiated in early Cambrian if not in pre-Cambriau time. It is evident that 

 we yet have much to learn of the medusiform ancestors of the Hydrozoa. 



FOSSIL MEDTJSJE OF THE MIDDLE CAMBRIAN TERRANE. 



As related in the Preface, a collection of fossils from the Coosa Valley, 

 Alabama, made in the summer of 1886, contained a number of semicherty 

 nodules, some of which had fragments of trilobites and brachiopods of the 

 Middle Cambrian fauna attached to and buried in them; others had a radi- 

 ate-lobed appearance that suggested the sea-urchin, while a few of the flat 

 nodules had a fossil spread out on them that resembled a star-fish. Large 

 collections were made during the succeeding years, but it was not until 

 1893 that I felt assured that the so-called "star-cobbles" were fossil medusa?. 

 There are now more than 9,000 specimens in the collections of the United 

 States Geological Survey. These afford ample material for the study of 

 two types 1 that may be referred to the Discomedusse. 



MODE OF OCCURRENCE. 



The shale containing the fossils breaks down into clay on exposure to 

 moisture, heat, and cold, and the siliceous nodules weather out from it and 

 are found in large numbers on the surface and along the drainage channels. 

 Fragments of trilobites, etc., occur in the shale, and are attached to and 

 embedded in many of the nodules, and, more rarely, attached to specimens 

 of the medusa?. The fossils common to the shale and nodules are: 



Laotira. cambria Walcott. Hyolithes. 



Acrotreta. Ptychoparia antiquata Salter. 



Lingulella. Ptychoparia. 3 sp. ( ?) 



Scenella. Olenoides curticei Walcott. 



Steuotheca. 



Of the nodules, about one-quarter show more or less of fossil medusa?. 

 A few of the larger flat nodules have several medusa? attached to each, but 

 usually a single individual forms the entire nodule, or serves as the nucleus 

 for a nodule that may vary in form and size from all its fellows. It is rare 

 to find two that agree in all respects. 



1 A notice of these appeared in the Proceedings of the United States National Museum, Vol. 

 XVIII, 1896, pp. 611-614. 



