156 



M. MOHANTI AND C.H.C. BRUNTON 



Type species. 

 designation. 



Genus ISOPOMA Torley, 1934 

 Terebratula brachyptycta Schnur 1 853, by original 



<0>* 



Diagnosis. Aseptirhynchid lacking adult dental plates, with dor- 

 sal valve thickening confined umbonally and free, disjunct hinge 

 plates extending from well-developed inner socket ridges. Ventral 

 valve thick-walled with prominent teeth. There are no median septa 

 in either valve. 



Comment. Below we describe or comment on the twelve species 

 described as belonging to lsopoma, including one new species. The 

 name /. brachyptyctum has been used in Germany to include speci- 

 mens resembling the lectotype of Schmidt (1941), which are longer 

 than wide, as well as more equidimensional specimens commonly 

 found in the Middle and Upper Givetian strata, typically to the east 

 of the river Rhine, Germany. We provide here some sections taken 

 from both forms but have insufficient information to be able to 

 differentiate these forms clearly, other than by their exteriors. Other 

 species, eg. /. orthoglossa (Torley) and /. ovale Xian & Jian, 

 somewhat resemble the wide forms of /. brachyptyctum and are of 

 similar age. Further studies of these faunas are required to resolve 

 these taxonomical problems and the stratigraphical range of true /. 

 brachyptyctum. 



Shell STRUCTURE. The shell structure of lsopoma (Figs 19-2 1 ) is 

 poorly known: no primary layer has been seen in fresh condition, but 

 the secondary fibres are relatively large and angular; being some- 

 what rhomboidal in section. This is characteristic of many Palaeozoic 

 rhynchonellids. 



lsopoma brachyptyctum (Schnur, 1853) 



Figs 1-5, 19 



1853 Terebratula brachyptycta Schnur: 178: pi. 23, fig. 6. 

 1934 lsopoma brachyptyctum (Schnur); Torley: 81, pi. 3, 

 12-15. 



figs 



la K 



Figs 1, 2 lsopoma brachyptyctum (Schnur). 1, syntype; copy of Schnur. 

 1853: pi. 23, fig. 6; presumed from Eifelian limestone, Blankenheim, 

 Germany; Schnur Collection, Geol.-Palaontological Institute, Bonn; x 2. 

 2, lectotype; a syntype from the Schnur Collection resembling that of 

 Fig. 1; copy of Schmidt 1941: pi. 4, fig. 88; locality unspecified, but 

 possibly Ahrdorf Formation, Eifelian, Germany; x 2. Both specimens 

 are illustrated in dorsal, anterior and ventral views, but note that 

 Schmidt illustrated the anterior view with the ventral valve uppermost. 



3a 



3c 



Fig. 3 lsopoma brachyptyctum (Schnur). Dorsal, ventral, anterior and 

 lateral views; Givetian Massenkalk. Bilveringsen, Germany; BD 12784, 

 x3. 



1941 lsopoma brachyptyctum (Schnur); Schmidt: 45, pi. 4, fig. 



88. 

 1966 lsopoma brachyptyctum (Schnur); Biernat: 106, pi. 22, figs 



1-9. 



Type specimen. The lectotype, selected by Schmidt ( 1941 : 45, pi. 

 4, fig. 88), is from Im Kalk zu Blankenheim (Schnur, 1853: 178). 

 Schmidt did not specify the precise stratigraphical level of this 

 specimen, which is in the Schnur Collection at the Geol.- 

 Palaontological Institute. Bonn, Germany. However, the lectotype 

 possibly came from the Ahrdorf Formation (Mohanti, 1972: 170), 

 which is considered to be Middle Eifelian in age (see Paulus, Struve 

 & Wolfart, 1963: 466, text-fig. 1). 



Diagnosis. Outline approximately equidimensional, with great- 

 est width at about two-thirds of the length of the shell; ventral 

 sulcation prominent, commonly containing two ribs and an addi- 

 tional pair of ribs lateral to the sulcus; fold weakly developed. 



Material. German specimens from the Eifel region are: 3 speci- 

 mens (SMF XVII 342b, XVII 342c and XVII Bi) from Bilveringsen; 

 one specimen (SMF XVII 1 168a) from Schleddenhof; one specimen 

 (SMF XVII 342k) from the Ahrdorf Formation at Gees; and 5 

 specimens (SMF XVII 2420) from the Ahbach Formation at Hallert. 

 In addition, unregistered specimens were kindly donated by the late 

 Dr Struve from the upper part of the Lahr Member of the Ahbach 

 Formation (Loc. St. 949) (BD 12782-83) and from the Massenkalk 

 (Kohlenstein Member) of the Kohlenstein quarry at Iserlohn- 

 Bilveringsen (BD 12784). 



Stratigraphical range. According to Schmidt (1941: 45), in 

 the Eifel area of Germany lsopoma brachyptyctum ranges from 

 Middle Eifelian (Ahrdorf Formation) to the Fleringen Schichten, 

 which is now known to range from the Ahbach Formation (upper- 

 most Eifelian) to the Curten Formation (Lower Givetian) (Paulus, 

 Struve & Wolfart, 1963: 466, text-fig. 1 ). The species is also known 

 to the east of the Rhine river in Germany. Torley ( 1934) described 

 lsopoma brachyptyctum (Schnur) from the Massenkalk (Upper 

 Givetian) of Bilveringsen, near Iserlohn. Schmidt ( 1 95 1 ) also briefly 

 remarked upon a specimen of lsopoma brachyptyctum from the 

 Upper Givetian Flinz kalk of Schleddenhof, near Iserlohn-Letmathe 

 on the east side of the Rhine. 



