72 ROBERT TRACY JACKSON ON ECHINI. 



p. 80) and spatangoids (Micraster and CoUyrites, Plate 3, fig. 15). In both of these the 

 primordial ambulacral plates typically alternate with the primordial interambulacral plates 

 to form the basicoronal row. In all other echinoids the primordial ambulacral plates are 

 on the actinostome and form buccal plates both in the young and in the adult. Some very 

 peculiar special modifications of this plan occur in aberrant cases in clypeastroids and spatan- 

 goids, as follows. 



In Arachnoides placenta (Linne), Loven (1874, Plate 51) showed that in the young, 

 ambulacral and interambulacral plates alternate as usual. In the adult, the posterior odd 

 interambulacral plate has been pushed dorsally out of the basicoronal ring, and the other 

 primordial interambulacral plates in areas 1, 2, 3, 4, have disappeared by intracoronal resorption, 

 so that as a result, the ten primordial ambulacral plates have come in contact and alone make 

 up the basicoronal ring. They are in contact as they are typically in the buccal membrane 

 around the mouth in regular Echini (Bothriocidaris), but the contact in Arachnoides placenta 

 is a secondary contact not homologous with, though analogous to the original primitive con- 

 tact. In specimens of Arachnoides that I have examined, both smaller and larger than those 

 figured by Loven, the primordial interambulacral plates are in place in the basicoronal row. 

 It is probable, therefore, that Loven's larger specimen as described was an exceptional indi- 

 vidual variant, and not typical of the species. 



In Lovenia forbesi Woods and Duncan, from the Miocene of Australia, the primordial 

 interambulacral plates of areas 2, 3, 5, are in place in the basicoronal row, but the plates of 1 

 and 4 have been pushed dorsally so that the primordial ambulacrals 16, Ila, and lYb, \a 

 have come in contact instead of being separated by an interambulacral plate as usual. 



In Pourtalesia jeffreysi Wyville Thomson, Loven (1883, p. 13) shows the extraordinary 

 condition that the primordial interambulacral plates 2, 3 join posteriorly the base of primordial 

 interambulacral 5 and shut out the primordial ambulacrals I, V and II, IV from the peristomal 

 row. In this highly aberrant type, therefore, the ventral margin of the corona is made up of 

 the two primordial ambulacrals Ilia, b, and the sum of primordial interambulacrals 2, 3, and 5, 

 as far as known a unique condition. Dr. Mortensen (1907, p. 73, Plate 17, fig. 21) shows in 

 Echinosigra (Pourtalesia) paradoxa Mortensen, a most excessive lengthening and also narrowing 

 of the basicoronal plates in areas I, 5, and V, making them, perhaps, the most peculiar plates 

 of any echinoid, in a type that takes delight in breaking all rules of echinoid structure. 



All the above are types of basicoronal plates in which there has been no encroachment 

 by resorption of the ventral border of the corona. Wliere resorption has taken place to a 

 greater or less extent, a different character of basicoronals prevails. 



In the Palaeechinidae resorption has apparently cut away only the single primordial 

 interambulacral plate and at the same time, as gathered from Melonechinus (Plate 56, fig. 8), 

 some ambulacral plates have been transferred to the peristome. In these types we find two 



