70 i. The Ohio Journal of Science [Vol. XIX, No. 1, 



much more primitive condition of these plates. Another 

 modification is shown in Figure 52 of Plate IX. Figure 54, 

 Plate IX is the diagram of the center of a specimen of A. 

 rectiradiatiis from Cherry Fork, Adams County, showing 

 the component parts of these peristomial plates as indicated by 

 lines of fusion on plates. 



The primitive condition of the peristomial region is best 

 shown in Stre piaster, a specimen from the Dyer collection. 

 In most specimens of Streptaster , since there is a great reduction 

 in diameter and length of the cover plates in the mouth region 

 and since the plates over the rest of the food groove are so 

 hypertrophied the peristomial region is rarely seen. Figure 

 47, Plate VIII, a diagram of the, specimen mentioned, shows the 

 small mouth plates with very little fusion into special plates. 

 The plate marked P. shows by its grooving that it is a union of 

 three simple plates. There is no way of distinguishing any 

 definite anterior plates as there are three plates where one would 

 expect the anterior peristomial plates R. and L. 



The large peristomial plates described above, then, are 

 made by fusion of a number of simple cover plates and may 

 include the nearest inter-radials as well. This fusion often 

 does not take place or at least does not take place completely 

 in many cases. 



PERISTOMIAL PLATES (ABORAL SIDE) AND PLATES FORMING 



SUBSTOMIAL CHAMBER. 



The substomial chamber has been described in detail by 

 Foerste from the Miller and Faber specimen of A. pileiis and 

 others. In most cases deformation of the specimen makes the 

 aboral pattern very hard to unravel. 



The material from Lawshe, which, for the purpose of this 

 discussion, I will call A. austini var. Lawshe is all exposed from 

 the aboral side and furnishes some additional information. 

 The aboral view of the large peristomial plate P in the posterior 

 inter-radial space has been obtained in several specimens. See 

 Plate II, Figure 12 for plate out of its relative position. Foerste's 

 description of it as ridged like the letter W on its surface toward 

 the substomial cavity gives a good idea of that surface. This 

 plate is shown rather indistinctly in Figure 44, Plate VII as 

 two masses in the anal inter-radius. The portion of the plate 

 toward the anal pyramid consists of a heavy ridge extending 



