STRUCTURE OF THE INTERAMBULACRUM. 143 



quite frequently specimens of Melonites multiporus are seen, showing three 

 plates ventrally. Such areas are to be considered incomplete ventrally, 

 and when well preserved show at the oral termination angles for the 

 articulation of the lowermost missing row. Another evidence of the 

 limits of the area ventrally is a line, M, of thin, dark colored, calcareous 

 tissue, which runs around the outer border of the peristome, coinciding 

 with the ventral edges of the plates. This apparently is the ventral 

 border of the perignathic girdle, or the auricles and other perpendicular 

 calcareous supports on the edge of the peristome for the attachment of 

 buccal muscles. This line of tissue stands out clearly in the open space 

 if the lowest row in any area is wanting, as between the areas C to H, in- 

 clusive (plate 2, figure 2). This perignathic girdle has only been observed 

 in this specimen. Besides the specimens figured, the ventral termination 

 of the interambulacra in two plates has been seen in a number of other 

 specimens of Melonites multiporus, amounting to 21 areas in all. In speci- 

 men number 3003, in the collections of the Museum of Comparative 

 Zoology, there are two plates at the ventral termination in all five inter- 

 ambulacral areas, as shown in plate 2, figure 3. These plates are suc- 

 ceeded by three plates in the second row. The same specimen shows 

 well the ventral termination of the ambulacral areas as seen in the figure. 

 Two specimens from the Boston Society of Natural History (catalogue 

 numbers 229A and 229B), also specimens in the E. M. Museum at Prince- 

 ton (catalogue numbers 1464 and 1466), show two plates ventrally in 

 several areas. 



This fact of two ventral plates is important because of the similar 

 number of columns of plates in the Euechinoidea. It is also important 

 as compared with the number of plates seen ventrally in other Paleozoic 

 echinoids (OUgoporus, Lepidechinus, Archdeocidaris). Its special bearing- 

 is the fact that it shows the limits of the encroachment of the peristome 

 on the corona of this genus, which may be taken as the type of the 

 family.^ The extent of encroachment varies in different types from not 

 at all, Bothriocidaris, Lepidechinus, to an extensive encroachment, Archmoci- 

 darls, Cldaris (see figures 42, 43, 44, 48 and 55 in the plates). 



The only published observations seen on the extreme ventral area of 

 Melonites is Meek and Worthen's (30) figure of the ventral aspect of Melo- 

 nites multiporus, showing the jaws in place. In the text they do not de- 

 scribe or discuss the plates, but in the figure the five ambulacral ai'eas 

 show four plates at their ventral termination. One of the interambulacra 

 has two plates ventrally, three have three plates, and one has five. This 

 discrepancy with our observations is unquestionably due to imperfections 

 of the specimen figured, not to variations in this important feature. 



* See classification at end of succeeding paper: Studies of Paleeecliinoidea. 

 XX— Bull. Geol. Soc. Am., Vol. 7, 1895. 



