ACROTRETID^. 633 



side (PL LVII, fig. 3b) . It is the same type of surface as that of Botsfordia pulchra (Matthew) 

 (PI. LXII, figs. 5, 5a-c) and Micromitra (Iphidella) pannula (White) (PL IV, figs. It arxd Is). 

 This form of surface is represented by only one species: Acrothele bellapundata Walcott 

 (PL LVII, figs. 3, 3a-b). 



The surface of the following species has not been definitely determined owing to the condi- 

 tion of preservation. I think most of them will be found to belong to the group represented 

 by A. coriacea Linnarsson (p. 642) : 



Acrothele dichotoma Walcott. 



prlmseva (de Verneuil and Barrande). 

 rara Walcott. 

 villaboimensis Delgado. 

 sp. undt. a Walcott. 

 sp. undt. Moberg. 



The group of forms described by G. F. Matthew [1903, pp. 98-103] from the "Etcheminian" 

 of Nova Scotia, and A. prima (Matthew) [1886, p. 41] from New Brunswick, and A. prima 

 costata (Matthew) [1895a, p. 128], are all closely related by their general form, usually strong shells, 

 and surface characters; the latter are variable but the range of variation is usually to be found 

 on any large, old shell, an exception being found in radiating ridges, which vary greatly and to a 

 certain extent depend upon conditions that affected the shell after the animal died, such as 

 maceration and distortion by compression and movement in the embedding sediment. The 

 shells above the horizon of the Paradoxides zone, represented by Acrothele matthewi (Hartt), 

 appear to have been affected also by the same conditions, although the sediment is usually 

 finer and more compact. 



Pompeckj [1896b, p. 603] thought that the shell described by de Verneuil and Barrande 

 [1860, pp. 536-537] was probably a species of Acrothele, and probably the same as the shell 

 collected by Miquel. My study of Miquel's material shows that a true Acrothele is repre- 

 sented: A. hergeroni Walcott (PL LVIII, figs. 6, 6a-c). The other form is biconvex, calcareous, 

 and not a typical Acrothele. I have referred to it provisionally (p. 602) ks Botsfordia? harrandei. 



Comparisons wifJi other genera. — The relations between Acrothele and Acrotreta were recog- 

 nized by Linnarsson [1876, p. 20] and by all subsequent authors who have had occasion to 

 discuss the genus. Both genera have (a) a more or less elevated ventral valve with a pedicle 

 opening at the summit of a more or less distinctly defined false area; (b) a moderately convex 

 dorsal valve with somewhat similar interior markings; (c) small visceral areas in the ventral 

 valve; (d) corneous shells built up of thin layers. 



Acrothele differs from Acrotreta (a) in having a depressed ventral valve; (b) a faintly defined 

 false area in the ventral valve; (c) larger size; (d) greater variety and more highly ornamented 

 surface; (e) apparently more complex vascular markings; and (f) a difference in the position 

 of the main vascular sinuses; for the dorsal valve this is best seen by comparing Plate LX, figures 

 Ik and 1 1, Acrothele suhsidua (White), with Plate LXV, figures If and Ig. I have examined 

 many hundred specimens of the interior of dorsal valves of Acrotreta without finding any traces of 

 the main vascular sinus, whereas in Acrothele they are usually present. In the ventral valve the 

 differences are also marked, as the vascular sinuses are proportionally much larger in Acrotreta. 



There are exceptions to the marked dift'erence in elevation of the ventral valve, as that 

 of Acrothele subsidua (Wliite) (PL LX, figs, la, Ic, and Id) is nearly as elevated as that of 

 Acrotreta sagittalis taconica (Walcott) (PL LXXI, figs. 1, la-e). 



Acrotreta appears to be a somewhat more highly delevoped form than Acrothele, although 

 a smaller shell. 



Geographic distribution. — Species of the Acrothele coriacea type occur in Sweden, Denmark, 

 Bohemia, southern France, and Portugal in Europe; in eastern North America, in Nova Scotia 

 and New Brunswick; in western North America in Utah, Nevada, Idaho, and Montana. Tliis 

 indicates the wide distribution of one specific type, and other forms extend the distribution 



