ORTMANN : TERTIARY INVERTEBRATES. 57 



As to the genus Dieringia = Iheringiana, there remains only a single 

 character, by which it may be recognized : the considerable narrowing of 

 the interambulacral spaces toward the margin of the test (see Lahille's 

 generic diagnosis on p. 14). It is true, this character distinguishes the 

 Patagonian fossil from all the known forms of Echinarachnius as well as 

 Sctitella. Nevertheless I do not believe that it is of generic value, since 

 this narrowing of the interambulacra is exhibited by several other species, 

 only in a less pronounced way. Especially this is true of Echinarachnius 

 Parma of the Atlantic coast of the United States, of which I have several 

 hundred individuals at my disposal. Since this decrease of width of the 

 interambulacra is brought about by an increase of width of the ambulacra, 

 and the latter is shown in all species of Scutella and Echinarachnius, we 

 may put it this way, that in Sciitella the ambulacral plates increase sud- 

 denly in width from the end of the petals toward the margin, and in the 

 Patagonian form this increase is most pronounced, so as to render the 

 interambulacra very narrow on the margin, while in other species this in- 

 crease goes only so far as to keep the interambulacra at the same width 

 from the end of the petals to the margin. Sometimes it causes even a 

 slight decrease in width: I have found a slight narrowing of the inter- 

 ambulacra in Ech. parma, Ech. excentricus (California), and very slightly 

 in specimens of Scut, interlineata Stps. (Gabb, 1869, p. no) from the 

 Pliocene beds of California. 



Lahille compares in this respect Iheringia with Monophora, and says 

 (p. 6 of separate copy) that in both this star-like form of the five inter- 

 ambulacra, with five sharp points of the pentagram, is very striking. But 

 comparing his figures of Monophora (Lahille, 1896, pi. 1-4, especially pi. 

 3, f. 36), there is no such close resemblance, Monophora being like Ech. 

 parma in this respect. I was able to confirm this fact by comparison of 

 an individual of Monophora darwini from the Territory of Chubut, sent 

 to us by v. Ihering. 



As Lahille points out, there can be distinguished, in Scutella patagonica, 

 two series of forms, one more regularly circular in outline ("mode rotun- 

 datus"), the other more dilated and transverse ("mode alatus"). Our 

 material also shows these two series, and I should like to make a few 

 remarks on them. 



We possess altogether 87 individuals, in which the outline is distinctly 

 recognizable. Out of this number only about 16 may be said to belong 



