ART. 21 ORDOVICIAN TEILOBITES ULRICH 43 



some of its most important structural features I have endeavored to 

 supply the desired information by devoting nearly an entire plate 

 to the illustration of its parts. The accurately figured specimens 

 shown in Plates 7 and 8 do not include either of the two complete 

 individuals that are comprised in the material before me but consist 

 mainly of separated parts of the dorsal shield that show many pre- 

 viously unrecorded details of structure. Most of these features are 

 mentioned in the descriptions of the figures in the plates to which the 

 reader is referred. However some of them deserve further discussion 

 here. 



To begin with I will call attention to the extremely spiny nature 

 of the surface of the cephalon and thorax. The pygidum, on the 

 contrary, has no spines but its surface is covered with very small 

 tubercules. Of course, as a rule only the broken stumps of these 

 spines are to be seen on the specimens as they show on the fractured 

 surface of the fine grained limestone matrix. It is only here and 

 there, as for instance at the left end of the occipital ring of the 

 cranidium in the left half of figure 2, that one gets an adequate con- 

 ception of the great length and extreme slenderness of many of these 

 spines. The length of those on the pleural parts of the thoracic 

 segments is shown on the right side of figure 1. Those on the axis 

 appear to be shorter. 



The ends of the thoracic segments, of which there are 10, are not 

 drawn out into recurved spines, as indicated in Whitfield's figure 

 of the thorax, but are simply turned sharply downward and terminate 

 somewhat bluntly and obliquely, the free edge being lined with a 

 fringe of minute spines, the posterior one of which is thicker and 

 much longer than the others. 



Another set or fringe of very small spines that seems to have been 

 overlooked previously occurs, as shown in figure 4, along the outer 

 edge of the free cheek. Regarding the free cheeks it should also be 

 observed that the two, as shown in figure 5, are connected across the 

 front by a narrow, parallel-sided, doublure-like band. However, 

 this band seems to be separated from the rim of the dorsal part of the 

 test by a suture along the anterior edge of the cephalon. The strong 

 arching of the edge- of the cephalon in the anterior view and the two 

 relatively strong spines on the anterior rim of the cranidium are 

 other noteworthy features. 



The facial suture cuts the posterior margin immediately behind 

 the base of the genal spine. In fact it seems to cut off a bit of the 

 base of the spine. Anteriorly it cuts the rim just outside of the 

 lateral extremity of the glabella. 



Among many strictly specific characters is the fact that there are 

 constantly three transverse rows of spines on the preglabellar field. 



