NO. 9 NEW CAMBRIAN TRILOBITE FAUNULE — RASETTI 3 



where indistinct. Most of the fossils were concentrated in small 

 pockets. 



The fossiliferous outcrop is located 27 mm from the west edge 

 and 164 mm from the north edge of the East Chatham 7;|-minute 

 USGS quadrangle, scale 1 : 24,000 (USGS collections 3810, 4216; 

 writer's collection cs-4). The faunule from this bed is referred to 

 hereafter as the Acimetopus hilohatus faunule from one of the most 

 common and characteristic trilobites. 



The most common fossils are small inarticulate brachiopods, espe- 

 cially an acrotretid and less abundant linguloid forms. Trilobites are 

 also copious in some portions of the rock, but most of the specimens 

 are too fragmentary for identification. Evidently the tests were trans- 

 ported by currents for considerable time and distance before being 

 covered by the sediment. All the trilobites are dismembered, except 

 one cephalon with two thoracic segments attached. Nevertheless, 

 many of the compact eodiscid cephala and pygidia did not suflfer 

 excessive damage. In exposing the fossils, the matrix invariably sepa- 

 rates at the boundary of the outer surface of the test, hence orna- 

 mentation is well preserved. Some of the specimens suffered slight 

 flattening or fracturing in the slumping or compaction of the sedi- 

 ment. Distortion due to tectonic deformation of the rock is present 

 to a slight degree in some of the material. 



The size of the trilobites presents an unusual situation. The Eodis- 

 cids are, on the average, large for the family. Examples of Litometo- 

 pus longispinus indicate that the species attained a length of 50 mm, 

 almost gigantic for an Eodiscid. Most of the examples are much 

 smaller, but very small, immature specimens seem to be lacking, 

 cephala or pygidia below 3 or 4 mm in length being rare. All the 

 Olenellids on the contrary are of small size, fairly complete cephala 

 not exceeding 4-5 mm in length. It seems difficult to attribute this 

 fact to the delicate nature of the olenellid tests and their consequent 

 fragmentation, since no fragments indicating individuals of medium 

 or large sizes were observed. In the well-known conglomeratic lime- 

 stones at Schodack Landing and Troy, even though the trilobite 

 tests are excessively fragmentary and no large cephala have been 

 recovered, the presence of adult examples of Elliptocephala asa- 

 phoides is proved by the frequent occurrence of fragments of their 

 large pleurae. Nor can the lack of very small Eodiscids and large 

 Olenellids be attributed entirely to sorting by current action, since 

 very small brachiopods and relatively large Eodiscids are rather 

 common. 



