Newitt, A., A. J. German, et al. (2008). "Congenital abnormalities of the feline vertebral column." Vet Radiol Ultrasound 49(1): 35-41.
In this retrospective study, radiographs from 200 cats were assessed to determine the incidence of congenital abnormalities of the spinal column. Congenital abnormalities were found in 46 cats (23%). No cats were identified with hemivertebrae or spina bifida, although block vertebrae were found in three cats. Transitional abnormalities were the most common, such as thoracicization of L1 (18 cats; 9%) and extra ribs on L1. Extra ribs were often smaller and fragmented and could be confused with rib fractures.
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Bailey, C. S. and J. P. Morgan (1992). "Congenital spinal malformations." Vet Clin North Am Small Anim Pract 22(4): 985-1015.
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