938 MR. T. H. WITHERS ON 



aS*. tuberculatum, a more specialized form of scuttjm, the umbo 

 being subcentral. On account of its higher evolutionary deve- 

 lopment, S. tuherculat^iinis placed in anew subgenus of Calantica, 

 namely Titanolepas, and it is here suggested that this form may 

 represent an offshoot from the ancestral Scillcelepas line, and may 

 eventually have given rise to the genus Oxynaspis. 



Both species are remarkable in having an advanced form of 

 scutum in which the umbo is subcentral, and, together with a 

 small undescribed species of which only the carina is known, 

 constitute the oldest known examples of this specialized type of 

 valve. It is quite evident that Zeugviatolepas Qiiockleri and 

 Titanolepas tuherctdata are derived from either the genus 

 PoUicipes or from a Pollicipes-\\kQ ancestor, and both have 

 evolved the same type of scutum. These forms seem to show 

 tha.t the transition of the umbo in the scutum from an apical 

 to a subcentral position, has occurred independently in unrelated 

 forms belonging to distinct lines of development, and recent 

 work leads me to believe that this is equally true also of the 

 carina in the more advanced forms of Scalpellum (sensu, lato). 

 In T. tuherctdata the position of the umbo in the scutum is 

 constant ; but in Z. mockleri it changes apparently according to 

 age, although some individuals seem to be more advanced in this 

 respect. It may be, therefore, that in the scutum of Z. mockleri 

 the subcentral umbo was quite a recent development. 



The two species must have been comparatively common in the 

 Chalk sea, for of Zeugmatolepas mockleri we have remains of at 

 least 100 capitula, and of Titanolepas tuherctdata more than 30. 



Family PoLLiciPEDiDiE. 

 Zeugmatolepas *, gen. nov. 



Capitulum longer than broad, comprising at least 34 valves, 

 composed of scuta, terga, upper latera, and carina, and three or 

 more whor-ls of subtriangular lower latera, with a rostrum, 

 subrostrum, and subcarina ; upper latera almost as large as the 

 scuta, occupying the whole of the space between the scuta and 

 terga., the apices of the upper latera contiguous with those of the 

 scuta ; umbo of scuta subcentral, and of the remaining valves 

 apical. 



Genotype. — Zeugmatolepas mocTcleri. 



Zeugmatolepas mockleri, sp. n. (Plate XCIY. figs. 1-14.) 

 Diagnosis. — Yalves thin and generally smooth. Caiina narrow, 

 usually with a sti-ong, median, rounded keel, not divided ofl" into 

 parietes and intraparietes, basal margin acutely angular. Scutum 

 subtriangular to trapezoidal in shape, with the umbo varying in 

 position from just below the apex to about one-third the length 

 of the valve from the apex ; there is in most scuta a deep trough 



* ZfVYfxa = a connecting link. 



